Re: [PATCH] tty: serial: Use fifo in 8250 console driver

[Wander Costa <wcosta@redhat.com>]

On Tue, Jan 25, 2022 at 6:36 AM Jiri Slaby <jirislaby@kernel.org> wrote:
>
> On 25. 01. 22, 10:08, Jiri Slaby wrote:
> > On 25. 01. 22, 9:39, Jon Hunter wrote:
> >>
> >> On 29/10/2021 21:14, wander@redhat.com wrote:
> >>> From: Wander Lairson Costa <wander@redhat.com>
> >>>
> >>> Note: I am using a small test app + driver located at [0] for the
> >>> problem description. serco is a driver whose write function dispatches
> >>> to the serial controller. sertest is a user-mode app that writes n bytes
> >>> to the serial console using the serco driver.
> ...
> >>> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> >>
> >>
> >> On the current mainline and -next branches, I have noticed that the
> >> serial output on many of our Tegra boards is corrupted and so
> >> parsing the serial output is failing.
> >>
> >> Before this change the serial console would appear as follows ...
> >>
> >> [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x411fd071]
> >> [    0.000000] Linux version 5.16.0-rc6-00091-gadbfddc757ae
> >> (jonathanh@jonathanh-vm-01) (aarch64-linux-gnu-gcc (Linaro GCC
> >> 6.4-2017.08) 6.4.1 20170707, GNU ld (Linaro_Binutils-2017.08)
> >> 2.27.0.20161019) #15 SMP PREEMPT Tue Jan 25 00:15:25 PST 2022
> >> [    0.000000] Machine model: NVIDIA Jetson TX1 Developer Kit
> >>
> >> And now I see ...
> >>
> >> [    0.000000] Booting Linux on physicalfd071]
> >> [    0.000000] Linux version 5.16.0-rc6-athanh@j-linux-g017.08)
> >> Linaro_B20161019n 25 00:[    0.000000] Machine model: NVIDIA Jet[
> >> 0.000000] efi: UEFI not found.
> >> [    0.000000] NUMA: No NUMA configurati[    0.000000] NUMA: Faking a
> >> node at [m00000001[    0.000000] NUMA: NODE_DATA [mem 0x17[
> >> 0.000000] Zone ranges:
> >>
> >> Bisecting is pointing to this commit. Let me know if there are any
> >> tests I can run. Otherwise we may need to disable this at least
> >> for Tegra.
> >
> >
> > The test is bogus:
> >          use_fifo = (up->capabilities & UART_CAP_FIFO) &&
> >                  port->fifosize > 1 &&
> >                  (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO)
> >
> > FCR is write only. Reading it, one gets IIR contents.
>
> In particular, the test is checking whether there is no interrupt
> pending (UART_FCR_ENABLE_FIFO == UART_IIR_NO_INT). So it oscillates
> between use_fifo and not, depending on the interrupt state of the chip.
>
> Could you change it into something like this:
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -3396,7 +3396,7 @@ void serial8250_console_write(struct
> uart_8250_port *up, const char *s,
>
>          use_fifo = (up->capabilities & UART_CAP_FIFO) &&
>                  port->fifosize > 1 &&
> -               (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO) &&
> +               (up->fcr & UART_FCR_ENABLE_FIFO) &&
>                  /*
>                   * After we put a data in the fifo, the controller will
> send
>                   * it regardless of the CTS state. Therefore, only use fifo
>

Indeed I made a mistake here. Independent of the reported this, this
should be fixed.
Jiri, do you intend to send an official patch or should I do so?

>
> And see whether it fixes the issue. Anyway, of what port type is the
> serial port (what says dmesg/setserial about that)?
>
> thanks,
> --
> js
> suse labs
>