Raspberrypi3: Mini UART driver
Niteesh
gsnb.gn at gmail.com
Tue Dec 24 18:07:23 UTC 2019
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>
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>> 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>> 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>>> 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>>>
> 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>>> 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>>>> 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>>>> 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>>
> > > <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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