linux-kernel - Re: [PATCH] serial: tegra: Fixes lower tolerance baud rate limit for older tegra chips introduced by d781ec21bae6

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Date:   Sun, 28 Nov 2021 00:19:43 +0100
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Patrik John <patrik.john@...lox.com>
Cc:     linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        ldewangan@...dia.com, thierry.reding@...il.com,
        jonathan@...dia.com, linux-serial@...r.kernel.org,
        linux-tegra@...r.kernel.org
Subject: Re: [PATCH] serial: tegra: Fixes lower tolerance baud rate limit for
 older tegra chips introduced by d781ec21bae6

On Mon, Nov 22, 2021 at 01:44:26PM +0100, Patrik John wrote:
> The current implementation uses 0 as lower limit for the baud rate tolerance which contradicts the initial commit description (https://git.kernel.org/pub/scm/linux/kernel/git/tegra/linux.git/commit/drivers/tty/serial/serial-tegra.c?h=for-next&id=d781ec21bae6ff8f9e07682e8947a654484611f5) of +4/-4% tolerance for older tegra chips other than Tegra186 and Tegra194.
> This causes issues on UART initilization as soon as the actual baud rate clock is slightly lower than required which we have seen on the Tegra124-based Toradex Apalis TK1 which also uses tegra30-hsuart as compatible in the DT serial node (for reference line 1540ff https://git.kernel.org/pub/scm/linux/kernel/git/tegra/linux.git/tree/arch/arm/boot/dts/tegra124-apalis-v1.2.dtsi?h=for-next)
> 
> The standard baud rate tolerance limits are also stated in the tegra20-hsuart driver description (https://www.kernel.org/doc/Documentation/devicetree/bindings/serial/nvidia%2Ctegra20-hsuart.txt).
> 
> The previously introduced check_rate_in_range() always fails due to the lower limit set to 0 even if the actual baud rate is within the required -4% tolerance.
> 
[...]

I have a same patch waiting in my tree [1]. Feel free to use the commit
message and to add:

Reviewed-and-tested-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>

[1] https://rere.qmqm.pl/git/?p=linux;a=commitdiff;h=b658dcd83d0db777410fe960721193d35a38115a

Best Regards
Michał Mirosław