Re: imx serial chars lost issue

[Sergey Organov <sorganov@gmail.com>]

Francesco Dolcini <francesco@dolcini.it> writes:

> On Wed, Oct 26, 2022 at 12:06:34AM -0300, Fabio Estevam wrote:
>> Hi Francesco,
>> 
>> On Mon, Oct 3, 2022 at 8:08 AM Francesco Dolcini
>> <francesco.dolcini@toradex.com> wrote:
>> >
>> > Hello all,
>> > recently I have been experiencing an issue with the imx uart on i.MX6ULL
>> 
>> Does this also happen with other i.MX devices or only on i.MX6ULL?
>
> I do not know for sure, I did not try to actively reproduce the issue on
> another SoC. On the other hand the issue with the i.MX6ULL was noticed
> during some testing, and we do exactly the same tests on other SoC and
> there the issue is either not present or just happening very
> infrequently.
>
>> > SoC, on the tty console (no flow control, 1152008n1), chars after the 32nd
>> > are lost when typing "fast" (copy/paste or writing to the uart using
>> > automated tools).
>> >
>> > This was tested on Linux 6.0, however it does not look like a brand new
>> > regression, kernel 5.4 was somehow subject to the same issue, although
>> > it was way more difficult to trigger it.
>> >
>> > While I do understand that without flow control this is something that
>> > cannot be prevented, it is pretty fishy that is always the chars after
>> > the 32nd.
>> >
>> > I guess that might be happening is that the FIFO is emptied only by a
>> > timer or when it is full, instead of emptying it when half-full, and the
>> > CPU might not be fast enough leading to an overflow and the chars lost.
>> >
>> > Any idea?
>> 
>> Can you share a reproducer?
>
> Just open the uart console with a terminal application and copy/paste the
> following:
>
>  echo -n "12345678901234567890123456789012345678901234567890123456789012345678901234567890" | wc
>
> This is supposed to count 80 chars, you'll get less.

I've tested this on my board featuring iMX6SX CPU, using socat on both
ends of the serial line:

On PC, using MOXA multi-port serial board:

$ echo -n "12345678901234567890123456789012345678901234567890123456789012345678901234567890" |  socat - /dev/ttyMI3,cfmakeraw,b115200

while running

$ socat -u /dev/ttymxc5,cfmakeraw,b115200 ./log

on the board, and I got exactly 80 bytes in the ./log.

The kernel running does not contain mentioned 7a637784d517 ("serial:
imx: reduce RX interrupt frequency") though, so maybe it's just that.

-- Sergey Organov