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

Stefan Wahren
2022-04-18T16:02:01Z

Am 18.04.22 um 13:38 schrieb Ivan T. Ivanov:
> On 04-18 13:22, Stefan Wahren wrote:
>> 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@suse.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 have workaranded this the same, at first, but then decided to look at
> vendor tree and voilà!
>
>> 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?
>>
> For (open)SUSE we use downstream DTS.

This is popular and bad at the same time. We as the mainline kernel
developer cannot guarantee that this works as expected. A lot of issues
are caused by mixing vendor DTS with mainline kernel, so in general (not
for this specific issue) you are on your own with this approach.

I know this is a little bit off topic but except from overlay support,
can you provide a list of most missing features of the mainline kernel /
DTS?

> Do you think that if I put better description in commit message fix will
> be more acceptable.
At least it would increase the chance to be accepted. This rounding
behavior looks open coded, maybe there is already a function to achieve
this.
> Or if someone could suggest anything else I am open
> to discussion.
>
> Regards,
> Ivan
>
>
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