Raspberrypi3: Mini UART driver
Niteesh
gsnb.gn at gmail.com
Tue Dec 24 11:06:55 UTC 2019
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.
On Tue, Dec 24, 2019 at 2:13 PM Niteesh <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>
> 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>> 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>> 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>> 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>>>
>> 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>>>
>> 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
>> > > >
>> > >
>> > > _______________________________________________
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>> > <mailto:devel at rtems.org <mailto:devel at rtems.org>>
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>> > >
>> >
>>
>
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