linux-kernel - Re: [PATCH v2] clk: bcm2835: Round UART input clock up

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Date:   Mon, 18 Apr 2022 13:22:44 +0200
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     "Ivan T. Ivanov" <iivanov@...e.de>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Phil Elwell <phil@...pberrypi.org>,
        kernel test robot <lkp@...el.com>,
        bcm-kernel-feedback-list@...adcom.com, linux-clk@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] clk: bcm2835: Round UART input clock up

Hi Ivan,

Am 18.04.22 um 13:05 schrieb Ivan T. Ivanov:
> Hi Stefan,
>
> On 04-15 10:52, Stefan Wahren wrote:
>> Hi Ivan,
>>
>> Am 14.04.22 um 12:56 schrieb Ivan T. Ivanov:
>>> Hi Stefan,
>>>
>>> Please, could you take a look into following patch?
>> yes, but i cannot give a technical review. But from my gut feeling this
>> doesn't look really elegant to me.
>>> Thanks!
>>> Ivan
>>>
>>> On 04-04 15:51, Ivan T. Ivanov wrote:
>>>> Subject: [PATCH v2] clk: bcm2835: Round UART input clock up
>>>> Message-Id: <20220404125113.80239-1-iivanov@...e.de>
>>>>
>>>> The UART clock is initialised to be as close to the requested
>>>> frequency as possible without exceeding it. Now that there is a
>>>> clock manager that returns the actual frequencies, an expected
>>>> 48MHz clock is reported as 47999625. If the requested baudrate
>>>> == requested clock/16, there is no headroom and the slight
>>>> reduction in actual clock rate results in failure.
>>>>
>>>> If increasing a clock by less than 0.1% changes it from ..999..
>>>> to ..000.., round it up.
>> Based on this commit message this looks like a fix / workaround for an
>> issue. It would be very helpful to know:
>>
>> What issue should be fixed?
>>
>> Why is it fixed here and not in the UART driver for instance?
> The UART driver is amba-pl011. Original fix, see below Github link,
> was inside pl011 module, but somehow it didn't look as the right
> place either. Beside that this rounding function is not exactly
> perfect for all possible clock values. So I deiced to move the hack
> to the platform which actually need it.
thanks for your explanation. These are import information which belongs 
in the commit log, because the motivation and the affected UART is very 
important.
>
>> In case it fixes a regression, a Fixes tag should be necessary.
> I found the issue because it was reported that RPi3[1] and RPi Zero 2W
> boards have issues with the Bluetooth. So it turns out that when
> switching from initial to operation speed host and device no longer
> can talk each other because host uses incorrect baud rate.

Now i remember this issue, for the mainline kernel we decide to 
workaround the issue by lowering the BT baudrate to 2000000 baud. I 
didn't investigate the issue further, but your approach is a better 
solution.

Do you use the mainline DTS or the vendor DTS to see this issue?

Best regards

> I open to better solution of the issue.
>
> Thanks,
> Ivan
>
>> In best case this is explained in the commit message.
>>
>> Best regards
>>
>>>> This is reworked version of a downstream fix:
>>>> https://github.com/raspberrypi/linux/commit/ab3f1b39537f6d3825b8873006fbe2fc5ff057b7
>>>>
> [1] https://bugzilla.suse.com/show_bug.cgi?id=1188238
>