linux-kernel - Re: [PATCH] serial: atmel: fix incorrect baudrate setup

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Date:   Mon, 9 Jan 2023 08:22:36 +0100
From:   Tobias Schramm <t.schramm@...jaro.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Richard Genoud <richard.genoud@...il.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: atmel: fix incorrect baudrate setup


Am 09.01.23 um 08:07 schrieb Greg Kroah-Hartman:
> On Mon, Jan 09, 2023 at 08:02:00AM +0100, Tobias Schramm wrote:
>> Commit ba47f97a18f2 ("serial: core: remove baud_rates when serial console
>> setup") changed uart_set_options to select the correct baudrate
>> configuration based on the absolute error between requested baudrate and
>> available standard baudrate settings.
>> Prior to that commit the baudrate was selected based on which predefined
>> standard baudrate did not exceed the requested baudrate.
>> This change of selection logic was never reflected in the atmel serial
>> driver. Thus the comment left in the atmel serial driver is no longer
>> accurate.
>> Additionally the manual rounding up described in that comment and applied
>> via (quot - 1) requests an incorrect baudrate. Since uart_set_options uses
>> tty_termios_encode_baud_rate to determine the appropriate baudrate flags
>> this can cause baudrate selection to fail entirely because
>> tty_termios_encode_baud_rate will only select a baudrate if relative error
>> between requested and selected baudrate does not exceed +/-2%.
>> Fix that by requesting actual, exact baudrate used by the serial.
>>
>> Signed-off-by: Tobias Schramm <t.schramm@...jaro.org>
>> ---
> What commit id does this fix?  Please list that as a the "Fixes:" tag.

Commit ba47f97a18f2 ("serial: core: remove baud_rates when serial console
setup") is the one that breaks the assumptions made in the driver.
I'll send a v2 and include that.

>
> Also, does this need to go to older/stable kernels?
Yep,  this should go to older kernels as a fix, too.
>
> thanks,
>
> greg k-h