Raspberrypi3: Mini UART driver

Christian Mauderer list at c-mauderer.de
Thu Dec 26 20:29:46 UTC 2019


On 25/12/2019 15:32, Niteesh wrote:
> What about the framebuffer it still uses minor and major's?

I'm not entirely sure but I don't think that it is a problem to mix
legacy and new style drivers.

> 
> On Wed, Dec 25, 2019 at 3:20 PM Niteesh <gsnb.gn at gmail.com
> <mailto:gsnb.gn at gmail.com>> wrote:
> 
>     Just to make sure I am going in the right track.
>     I moved the uart register definitions to bsp/usart.h into a struct
>     of uint32_t called usart0_regs
>     here is git diff of usart.c after changing it to the latest console
>     interface.
> 
>     diff --git a/bsps/arm/raspberrypi/console/usart.c
>     b/bsps/arm/raspberrypi/console/usart.c
>     index 25fb523621..b12f375a1c 100644
>     --- a/bsps/arm/raspberrypi/console/usart.c
>     +++ b/bsps/arm/raspberrypi/console/usart.c
>     @@ -47,6 +47,12 @@ static uint32_t usart_get_baud(const console_tbl *ct)
>      }
>      #endif
>      
>     +typedef struct {
>     +  rtems_termios_device_context base;
>     +  const char *device_name;
>     +  volatile usart0_regs *regs;
>     +}uart0_context;
>     +
>      static void usart_set_baud(int minor, int baud)
>      {
>       /*
>     @@ -55,10 +61,17 @@ static void usart_set_baud(int minor, int baud)
>       return;
>      }
>      
>     -static void usart_initialize(int minor)
>     +static volatile usart0_regs
>     *rpi_uart_get_regs(rtems_termios_device_context *base)
>      {
>     -  unsigned int gpio_reg;
>     +  uart0_context *ctx;
>     +
>     +  ctx = (usart0_regs *) base;
>     +  return ctx->regs;
>     +}
>      
>     +static void usart_initialize(rtems_termios_device_context *base)
>     +{
>     +  unsigned int gpio_reg;
>        /*
>        ** Program GPIO pins for UART 0
>        */
>     @@ -75,67 +88,81 @@ static void usart_initialize(int minor)
>        usart_delay(150);
>        BCM2835_REG(BCM2835_GPIO_GPPUDCLK0) = 0;
>      
>     +  volatile uint32_t *uart_regs = rpi_uart_get_regs(base);
>     +
>        /*
>        ** Init the PL011 UART
>        */
>     -  BCM2835_REG(BCM2835_UART0_CR)   = 0;
>     -  BCM2835_REG(BCM2835_UART0_ICR)  = 0x7FF;
>     -  BCM2835_REG(BCM2835_UART0_IMSC) = 0;
>     -  BCM2835_REG(BCM2835_UART0_IBRD) = 1;
>     -  BCM2835_REG(BCM2835_UART0_FBRD) = 40;
>     -  BCM2835_REG(BCM2835_UART0_LCRH) = 0x70;
>     -  BCM2835_REG(BCM2835_UART0_RSRECR) =  0;
>     -
>     -  BCM2835_REG(BCM2835_UART0_CR)   = 0x301;
>     -
>     -  BCM2835_REG(BCM2835_UART0_IMSC) = BCM2835_UART0_IMSC_RX;
>     -
>     -  usart_set_baud(minor, 115000);
>     +  uart_regs->cr  = 0;
>     +  uart_regs->icr = 0x7ff;
>     +  uart_regs->imsc = 0;
>     +  uart_regs->ibrd = 1;
>     +  uart_regs->fbrd= 40;
>     +  uart_regs->lcrh= 0x70;
>     +  uart_regs->rsrecr= 0;
>     +  uart_regs->cr = 0x301;
>     +  uart_regs->imsc = BCM2835_UART0_IMSC_RX;
>     +  // usart_set_baud(minor, 115000);
>      }
>      
>     -static int usart_first_open(int major, int minor, void *arg)
>     +static bool usart_first_open(
>     +  rtems_termios_tty *tty,
>     +  rtems_termios_device_context *base,
>     +  struct termios *term,
>     +  rtems_libio_open_close_args_t *args
>     +)
>      {
>     -  rtems_libio_open_close_args_t *oc =
>     (rtems_libio_open_close_args_t *) arg;
>     -  struct rtems_termios_tty *tty = (struct rtems_termios_tty *)
>     oc->iop->data1;
>     -  const console_tbl *ct = Console_Port_Tbl [minor];
>     -  console_data *cd = &Console_Port_Data [minor];
>     +  rtems_status_code sc;
>     +  uart0_context *ctx;
>     +  bool ok;
>      
>     -  cd->termios_data = tty;
>     -  rtems_termios_set_initial_baud(tty, ct->ulClock);
>     +  ctx = (uart0_context *) base;
>      
>     -  return 0;
>     +  usart_initialize(base);
>     +
>     +  sc = rtems_termios_set_initial_baud(tty,  USART0_DEFAULT_BAUD);
>     +  if ( sc != RTEMS_SUCCESSFUL ){
>     +    printk("Error setting the baud for termios\n");
>     +    return false;
>     +  }
>      }
>      
>     -static int usart_last_close(int major, int minor, void *arg)
>     +static int usart_last_close(
>     +  rtems_termios_tty *tty,
>     +  rtems_termios_device_context *base,
>     +  rtems_termios_open_close_args_t *arg)
>      {
>        return 0;
>      }
>      
>     -static int usart_read_polled(int minor)
>     +static int usart_read_polled(rtems_termios_device_context *base)
>      {
>     -  if (minor == 0) {
>     -    if (((BCM2835_REG(BCM2835_UART0_FR)) & BCM2835_UART0_FR_RXFE)
>     == 0) {
>     -       return((BCM2835_REG(BCM2835_UART0_DR)) & 0xFF );
>     -    } else {
>     -      return -1;
>     -    }
>     -  } else {
>     -    printk("Unknown console minor number: %d\n", minor);
>     -    return -1;
>     +  volatile usart0_regs *regs;
>     +
>     +  regs = rpi_uart_get_regs(base);
>     +
>     +  if ((regs->fr & BCM2835_UART0_FR_RXFE) == 0) {
>     +    return (regs->dr & 0xFF);
>        }
>     +
>     +  return -1;
>      }
>      
>     -static void usart_write_polled(int minor, char c)
>     +static void usart_write_polled(rtems_termios_device_context *base,
>     char c)
>      {
>     -   while (1) {
>     -     if ((BCM2835_REG(BCM2835_UART0_FR) & BCM2835_UART0_FR_TXFF) == 0)
>     -       break;
>     -   }
>     -   BCM2835_REG(BCM2835_UART0_DR) = c;
>     +  volatile usart0_regs *regs;
>     +
>     +  regs = rpi_uart_get_regs(base);
>     +
>     +  while (1) {
>     +    if (((regs->fr) & BCM2835_UART0_FR_TXFF) == 0)
>     +      break;
>     +  }
>     +  regs->dr = c;
>      }
>      
>      static ssize_t usart_write_support_polled(
>     -  int minor,
>     +  rtems_termios_device_context *base,
>        const char *s,
>        size_t n
>      )
>     @@ -143,7 +170,7 @@ static ssize_t usart_write_support_polled(
>        ssize_t i = 0;
>      
>        for (i = 0; i < n; ++i) {
>     -    usart_write_polled(minor, s [i]);
>     +    usart_write_polled(base, s[i]);
>        }
>      
>        return n;
>     @@ -154,14 +181,11 @@ static int usart_set_attributes(int minor,
>     const struct termios *term)
>        return -1;
>      }
>      
>     -const console_fns bcm2835_usart_fns = {
>     -  .deviceProbe = libchip_serial_default_probe,
>     -  .deviceFirstOpen = usart_first_open,
>     -  .deviceLastClose = usart_last_close,
>     -  .deviceRead = usart_read_polled,
>     -  .deviceWrite = usart_write_support_polled,
>     -  .deviceInitialize = usart_initialize,
>     -  .deviceWritePolled = usart_write_polled,
>     -  .deviceSetAttributes = usart_set_attributes,
>     -  .deviceOutputUsesInterrupts = false
>     -};
>     +const rtems_termios_device_handler bcm2835_uart0_handler_polled = {
>     +  .first_open = usart_first_open,
>     +  .last_close = usart_last_close,
>     +  .poll_read = usart_read_polled,
>     +  .set_attributes = usart_set_attributes,
>     +  .write = usart_write_support_polled,
>     +  .mode = TERMIOS_POLLED
>     +}
>     \ No newline at end of file
> 
>     On Wed, Dec 25, 2019 at 12:36 AM Joel Sherrill <joel at rtems.org
>     <mailto:joel at rtems.org>> wrote:
> 
> 
> 
>         On Tue, Dec 24, 2019, 12:19 PM Niteesh <gsnb.gn at gmail.com
>         <mailto:gsnb.gn at gmail.com>> wrote:
> 
>             And also the register definitions are in raspberrpi.h file
>             should I move them to usart.h.
> 
> 
>         Sounds right if you mean bsp/usart.h
> 
>             I have a doubt we have a register field in device_context
>             typedef struct { 
> 
>                 rtems_termios_device_context base; 
> 
>                 const char *device_name; 
> 
>                 volatile some_chip_registers *regs;
> 
>             } my_driver_context;
> 
>             How does the reg field point to the correct memory location?
>             for instance in IMX BSP,
>             there is a struct with register field's but none of the
>             define a memory location?
> 
> 
>         Make sure the structure has volatiles and proper alignment. :)
> 
> 
>             On Tue, Dec 24, 2019 at 11:37 PM Niteesh <gsnb.gn at gmail.com
>             <mailto:gsnb.gn at gmail.com>> wrote:
> 
>                 How to handle different serial devices? In other BSPs
>                 the uart devices are the same, so
>                 they were able to put it under a single array? But here
>                 we have 2 uarts and a FB?
> 
> 
>                 On Tue, Dec 24, 2019 at 8:18 PM Christian Mauderer
>                 <list at c-mauderer.de <mailto:list at c-mauderer.de>> wrote:
> 
>                     On 24/12/2019 12:06, Niteesh wrote:
>                     > The current raspi console section is like this:
>                     > The bsp_console_select in console_select.c is
>                     responsible for selecting
>                     > between uart and the framebuffer. It does so
>                     > by setting the Console_port_minor.
>                     > The console_config is responsible for output_char
>                     function.
>                     > And other files are driver code.
>                     > If rewriting, this would be my approach,
>                     > Rewrite the bsp_console_select to set some kind of
>                     a variable like in
>                     > IMX, then in console_initialize function
>                     > link the right driver to /dev/console.
>                     > Replace the console_tbl with the device_context
>                     and console_fns with
>                     > termios_device_handlers and
>                     > finally add in the console_initialization function.
> 
>                     I agree that this would be a clean solution. So if
>                     you want you can do
>                     that. But there might is a hurdle: As far as I
>                     understood you you only
>                     have a Pi3? So you might have a hard time testing
>                     the changes. Maybe the
>                     simulator could work.
> 
>                     Another possibility could be to set the
>                     "Console_port_minor" to
>                     something unused (for example -1). In that case you
>                     can define another
>                     /dev/console.
> 
>                     Best regards and merry Christmas (in case you celebrate)
> 
>                     Christian
> 
>                     >
>                     > On Tue, Dec 24, 2019 at 2:13 PM Niteesh
>                     <gsnb.gn at gmail.com <mailto:gsnb.gn at gmail.com>
>                     > <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>>> wrote:
>                     >
>                     >     Thank you so much, for such a detailed answer.
>                     Now things make
>                     >     really good sense to me, 
>                     >     going through the code now is just a breeze.
>                     But I still have one
>                     >     question
>                     >     for the newer driver interface is
>                     console_initialize the function
>                     >     which RTEMS calls while initializing
>                     >     the console? Which means I can't mess with the
>                     name right? It is
>                     >     similar to the main function, right?
>                     >
>                     >     The current driver is a legacy one, how do you
>                     want me to proceed,
>                     >     shall I rewrite the legacy to a
>                     >     the new one, this is will be a great
>                     learning experience for me also
>                     >     and we also get the BSP updated to the latest
>                     interface.
>                     >
>                     >
>                     >     On Tue, Dec 24, 2019 at 3:20 AM Christian Mauderer
>                     >     <list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>
>                     <mailto:list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>>> wrote:
>                     >
>                     >         Hello Niteesh,
>                     >
>                     >         quite a lot of questions. I'll try to
>                     answer them. Note that it
>                     >         has been
>                     >         some time since I had a detailed look at
>                     that code so if something I
>                     >         tell seems odd please don't hesitate to
>                     question it.
>                     >
>                     >         Please note that in RTEMS their are more
>                     or less two "levels" of
>                     >         support
>                     >         for a serial console:
>                     >
>                     >         1. A very basic polled system console
>                     (also known as
>                     >         "debug-console" in
>                     >         some BSPs). This one is used for printk
>                     and should work in basically
>                     >         every case. It is used for critical system
>                     messages like
>                     >         printing the
>                     >         exception frame. For that a BSP has to
>                     provide a
>                     >         "BSP_output_char" function.
>                     >
>                     >         2. A full featured UART driver integrated
>                     into Termios. That one
>                     >         will be
>                     >         used for all normal I/O on the UARTs.
>                     >
>                     >         As far as I know the "console_tbl
>                     Console_Configuration_Ports"
>                     >         belongs
>                     >         to a table based legacy interface. It is
>                     handled in the file
>                     >         bsps/shared/dev/serial/legacy-console.c.
>                     I'm not sure whether it is
>                     >         documented in the BSP guide because it
>                     shouldn't be used for new
>                     >         BSPs.
>                     >         Same is true for the "major" and "minor"
>                     stuff: It's not really
>                     >         used for
>                     >         new drivers.
>                     >
>                     >         Newer drivers use the initialization that
>                     is described in the manual
>                     >         that you have already found. Basically
>                     they use
>                     >         "rtems_termios_device_install" to register
>                     a new UART as
>                     >         "/dev/ttySomething". Some recent (ARM)
>                     BSPs that do that are the
>                     >         imx or
>                     >         the atsam.
>                     >
>                     >         The console that is used for stdin, stdout
>                     and stderr (printf,
>                     >         scanf,
>                     >         ...) is the one called "/dev/console"
>                     (defined in
>                     >         CONSOLE_DEVICE_NAME).
>                     >         For the legacy table based interface it's
>                     the one with the index of
>                     >         "Console_Port_Minor".
>                     >
>                     >
>                     >         If you want to access any UART other than
>                     the one for stdin and
>                     >         stdout
>                     >         you do that the same way like on Linux:
>                     Just use the "open"
>                     >         function on
>                     >         the "/dev/ttySomething" and use "read",
>                     "write" and simmilar or use
>                     >         "fopen" together with "fread", "fwrite",
>                     "fprintf", ...
>                     >
>                     >
>                     >         "printf" (and family) is a function
>                     belonging to the C library.
>                     >         In our
>                     >         case that's newlib. It will format your
>                     message and after some other
>                     >         preprocessing will call the "write"
>                     function of the file that is
>                     >         opened
>                     >         as stdout (which is "/dev/console" in the
>                     default case).
>                     >
>                     >
>                     >         I hope that I helped you with that
>                     explanation. Please feel free
>                     >         to ask
>                     >         anything if it isn't clear.
>                     >
>                     >         Best regards
>                     >
>                     >         Christian
>                     >
>                     >         On 23/12/2019 19:50, Niteesh wrote:
>                     >         > And finally, how does printf work? It is
>                     a macro? In that
>                     >         case, how does
>                     >         > any write to
>                     >         > a console work?
>                     >         >
>                     >         > On Tue, Dec 24, 2019 at 12:18 AM Niteesh
>                     <gsnb.gn at gmail.com <mailto:gsnb.gn at gmail.com>
>                     >         <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>>
>                     >         > <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com> <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>>>> wrote:
>                     >         >
>                     >         >     Is the correct port minor number set
>                     during the
>                     >         initialization? What
>                     >         >     is the application want's to
>                     >         >     access some other port?
>                     >         >
>                     >         >     On Tue, Dec 24, 2019 at 12:16 AM Niteesh
>                     >         <gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com> <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>>
>                     >         >     <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com> <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>>>> wrote:
>                     >         >
>                     >         >         I would like to clarify my
>                     doubts regarding the
>                     >         console driver.
>                     >         >         I went through the documentation
>                     >         >         for the console
>                     >         >       
>                     >       
>                       driver https://docs.rtems.org/branches/master/bsp-howto/console.html#introduction.
>                     >         >         But it is quite different from
>                     how some BSPs initialize.
>                     >         >         Correct me if I am wrong
>                     >         >         The console_tbl contains the
>                     various entries of serial
>                     >         ports.
>                     >         >         The console_fns is a struct of
>                     function pointers,
>                     >         which point to
>                     >         >         the BSP uart functions.
>                     >         >         The
>                     BSP_output_char_function_type is what will be
>                     >         called for
>                     >         >         printing a char on to the console.
>                     >         >         How does RTEMS initialize the
>                     uart? It's seems not to
>                     >         be same
>                     >         >         for all BSPs.
>                     >         >         The doc says that the driver's
>                     initialization function
>                     >         is called
>                     >         >         once during the rtems
>                     initialization process.
>                     >         >         The console init function
>                     install the serial driver using
>                     >         >         rtems_termios_device_install but
>                     there seems to be
>                     >         >         no such function in the
>                     raspberry pi? But there is a
>                     >         entry in
>                     >         >         console_fns for init function,
>                     but then how does it
>                     >         >         gets called?
>                     >         >         And for BSP's with multiple
>                     serial's, the output function
>                     >         >         chooses the right serial using
>                     console_port_minor,
>                     >         >         Is it during initialization?
>                     >         >         What is the need for get and set
>                     register functions? 
>                     >         >
>                     >         >         On Mon, Dec 23, 2019 at 1:04 AM
>                     Christian Mauderer
>                     >         >         <list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>
>                     <mailto:list at c-mauderer.de <mailto:list at c-mauderer.de>>
>                     >         <mailto:list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>
>                     <mailto:list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>>>> wrote:
>                     >         >
>                     >         >             On 22/12/2019 19:45, Joel
>                     Sherrill wrote:
>                     >         >             >
>                     >         >             >
>                     >         >             > On Sun, Dec 22, 2019,
>                     12:29 PM Niteesh
>                     >         <gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com> <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>>
>                     >         >             <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com> <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>>>
>                     >         >             > <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>
>                     >         <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>>
>                     <mailto:gsnb.gn at gmail.com <mailto:gsnb.gn at gmail.com>
>                     >         <mailto:gsnb.gn at gmail.com
>                     <mailto:gsnb.gn at gmail.com>>>>> wrote:
>                     >         >             >
>                     >         >             >     On Sun, Dec 22, 2019
>                     at 8:44 PM Christian
>                     >         Mauderer
>                     >         >             >     <list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>
>                     >         <mailto:list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>>
>                     <mailto:list at c-mauderer.de <mailto:list at c-mauderer.de>
>                     >         <mailto:list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>>>
>                     >         >             <mailto:list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>
>                     >         <mailto:list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>>
>                     <mailto:list at c-mauderer.de <mailto:list at c-mauderer.de>
>                     >         <mailto:list at c-mauderer.de
>                     <mailto:list at c-mauderer.de>>>>> wrote:
>                     >         >             >
>                     >         >             >         Hello Niteesh,
>                     >         >             >
>                     >         >             >         thanks for doing
>                     that work.
>                     >         >             >
>                     >         >             >         On 22/12/2019
>                     12:10, Niteesh wrote:
>                     >         >             >         > The rpi1 and
>                     rpi2 use the PL011 UART,
>                     >         whereas,
>                     >         >             with RPI's
>                     >         >             >         equipped with
>                     >         >             >         >
>                     wireless/Bluetooth module, the PL011 is
>                     >         >             connected to the Bluetooth
>                     >         >             >         > module, and the
>                     mini UART is used as
>                     >         the primary
>                     >         >             UART.
>                     >         >             >
>                     >         >             >         In my opinion it
>                     would be great if you
>                     >         could use
>                     >         >             the FDT to
>                     >         >             >         distinguish
>                     >         >             >         between the
>                     boards. That should allow to add
>                     >         >             raspberry 3 (and
>                     >         >             >         maybe 4)
>                     >         >             >         support without
>                     adding another BSP. More
>                     >         BSPs mean
>                     >         >             a bigger
>                     >         >             >         maintenance
>                     >         >             >         effort for the
>                     RTEMS community.
>                     >         >             >
>                     >         >             >     Learning more about
>                     FDT is on my list for a long
>                     >         >             time.  I would love
>                     >         >             >     to work on that
>                     >         >             >     but I have almost no
>                     exp with FDT's.
>                     >         >             >     But another thing
>                     could also be done, in
>                     >         >             >   
>                      raspberrypi/start/bspstart.c we get the
>                     >         revision and
>                     >         >             >     model of the board
>                     using the mailbox. Every
>                     >         board has
>                     >         >             a unique id,
>                     >         >             >     which we could use to
>                     initialize
>                     >         >             >     the BSP. But using FDT
>                     seems to be a more
>                     >         elegant
>                     >         >             option, it is a
>                     >         >             >     lot of work I think,
>                     but we could take
>                     >         >             >     help from libbsd and
>                     linux I suppose. What
>                     >         do you think?
>                     >         >             >
>                     >         >             >
>                     >         >             > I think there are almost
>                     always two steps to a
>                     >         project
>                     >         >             like this: get it
>                     >         >             > to work and make it nice. :)
>                     >         >             >
>                     >         >             > If you fix the startup
>                     code to read the board
>                     >         revision and
>                     >         >             memory size,
>                     >         >             > you can get a working BSP
>                     that dynamically
>                     >         adapts to the
>                     >         >             models and
>                     >         >             > memory variations with
>                     minimal modifications. If
>                     >         you want
>                     >         >             to then
>                     >         >             > convert the BSP to FDT, it
>                     will be a LOT easier
>                     >         to debug
>                     >         >             with a working BSP.
>                     >         >             >
>                     >         >             > Plus you may be able to
>                     identify every variation
>                     >         point
>                     >         >             based on just the
>                     >         >             > model info. Then FDT is
>                     just a matter of
>                     >         switching the
>                     >         >             source of
>                     >         >             > some/all of the info.
>                     >         >             >
>                     >         >             > That would be my work plan
>                     anyway.
>                     >         >
>                     >         >             I agree with Joel that a
>                     secure development basis
>                     >         (also
>                     >         >             known as "hack")
>                     >         >             as a first step is a good
>                     idea. You maybe even
>                     >         just make the
>                     >         >             mini UART
>                     >         >             the default driver while you
>                     are developing. Then
>                     >         you can be
>                     >         >             sure that
>                     >         >             you have the right driver.
>                     >         >
>                     >         >             As soon as that works you
>                     can either change to the
>                     >         revision
>                     >         >             method or
>                     >         >             (better) to the FDT one and
>                     after that the patches
>                     >         can be
>                     >         >             merged. Using
>                     >         >             the FDT isn't that
>                     complicated. Basically you
>                     >         search for a
>                     >         >             node based on
>                     >         >             different parameters. For an
>                     example you can take
>                     >         a look at
>                     >         >             the imx BSP.
>                     >         >             In imx_uart_probe
>                     >         (bsps/arm/imx/console/console-config.c) a
>                     >         >             fdt node is
>                     >         >             searched and based on that a
>                     UART driver is used.
>                     >         But again:
>                     >         >             Follow
>                     >         >             Joels suggestion to start
>                     simple and secure.
>                     >         >
>                     >         >             >
>                     >         >             >         >
>                     >         >             >         >
>                     >         >             >       
>                     >         >           
>                     >       
>                        https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
>                     >         >             >         > But from the
>                     above doc (PAGE 10), the
>                     >         mini uart
>                     >         >             has 16550 like
>                     >         >             >         registers
>                     >         >             >         > and RTEMS
>                     already has the driver for it
>                     >         >             >         >
>                     bsps/shared/dev/serial/ns16550.c. But
>                     >         I am not
>                     >         >             sure how
>                     >         >             >         compatible they
>                     >         >             >         > are? Should a
>                     new driver be
>                     >         implemented from
>                     >         >             scratch or use
>                     >         >             >         ns16550 if
>                     >         >             >         > possible?
>                     >         >             >
>                     >         >             >         In general it's
>                     better to re-use
>                     >         existing code.
>                     >         >             That has multiple
>                     >         >             >         advantages:
>                     >         >             >
>                     >         >             >         - It reduces the
>                     maintenance effort.
>                     >         Fewer code
>                     >         >             means fewer work.
>                     >         >             >         - If you have
>                     multiple driver for the
>                     >         same or
>                     >         >             similar hardware
>                     >         >             >         it can
>                     >         >             >         happen that a bug
>                     is fixed in one but
>                     >         not the other.
>                     >         >             >         - It's simpler to
>                     find a hardware to
>                     >         test changes.
>                     >         >             >         - The driver
>                     becomes more universal with
>                     >         every new
>                     >         >             supported
>                     >         >             >         hardware.
>                     >         >             >         That increases the
>                     chance that it fits
>                     >         the next
>                     >         >             new hardware.
>                     >         >             >
>                     >         >             >         I'm sure there are
>                     some more if you ask
>                     >         someone else.
>                     >         >             >
>                     >         >             >     I do understand the
>                     issues, I just spent
>                     >         some time
>                     >         >             reading the
>                     >         >             >     driver code.
>                     >         >             >     I think we could most
>                     probably use it. I
>                     >         will take a
>                     >         >             closer look and
>                     >         >             >     will update.
>                     >         >             >
>                     >         >
>                     >         >             Great.
>                     >         >
>                     >         >             >          
>                     >         >             >
>                     >         >             >         >
>                     >         >             >         > Also, the core
>                     clock on which the
>                     >         PL011 is based
>                     >         >             on is changed
>                     >         >             >         in rpi3.
>                     >         >             >         > Rpi1 and 2 use
>                     250Mhz as the default
>                     >         clock but
>                     >         >             it was changed
>                     >         >             >         to 400Mhz
>                     >         >             >         > in Rpi3 and newer
>                     >         >             >
>                     >         >             >         Again: Would be
>                     great if that could be
>                     >         adapted
>                     >         >             based on FDT or by
>                     >         >             >         reading the right
>                     registers.
>                     >         >             >
>                     >         >             >         >
>                     >         >             >         > Few differences
>                     between PL011 and Mini
>                     >         uart
>                     >         >             >         > The mini UART
>                     has smaller FIFOs.
>                     >         Combined with
>                     >         >             the lack of
>                     >         >             >         flow control,
>                     >         >             >         > this makes it
>                     more prone to losing
>                     >         characters at
>                     >         >             higher baud
>                     >         >             >         rates. It
>                     >         >             >         > is also
>                     generally less capable than
>                     >         the PL011,
>                     >         >             mainly due to
>                     >         >             >         its baud
>                     >         >             >         > rate link to the
>                     VPU clock speed.
>                     >         >             >
>                     >         >             >         That shouldn't
>                     really be a problem for
>                     >         the system
>                     >         >             console.
>                     >         >             >
>                     >         >             >         >
>                     >         >             >         > The particular
>                     deficiencies of the
>                     >         mini UART
>                     >         >             compared to the
>                     >         >             >         PL011 are :
>                     >         >             >         >
>                     >         >             >         > No break detection
>                     >         >             >         > No framing
>                     errors detection
>                     >         >             >         > No parity bit
>                     >         >             >         > No receive
>                     timeout interrupt
>                     >         >             >         > No DCD, DSR, DTR
>                     or RI signals
>                     >         >             >         >
>                     >         >             >
>                     >         >             >   
>                      _______________________________________________
>                     >         >             >     devel mailing list
>                     >         >             >     devel at rtems.org
>                     <mailto:devel at rtems.org> <mailto:devel at rtems.org
>                     <mailto:devel at rtems.org>>
>                     >         <mailto:devel at rtems.org
>                     <mailto:devel at rtems.org> <mailto:devel at rtems.org
>                     <mailto:devel at rtems.org>>>
>                     >         >             <mailto:devel at rtems.org
>                     <mailto:devel at rtems.org> <mailto:devel at rtems.org
>                     <mailto:devel at rtems.org>>
>                     >         <mailto:devel at rtems.org
>                     <mailto:devel at rtems.org> <mailto:devel at rtems.org
>                     <mailto:devel at rtems.org>>>>
>                     >         >             >   
>                      http://lists.rtems.org/mailman/listinfo/devel
>                     >         >             >
>                     >         >
>                     >
> 


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