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

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Message-ID: <20220418110516.s7jxsfa3jl7aagrf@suse>
Date:   Mon, 18 Apr 2022 14:05:16 +0300
From:   "Ivan T. Ivanov" <iivanov@...e.de>
To:     Stefan Wahren <stefan.wahren@...e.com>
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 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.

> 
> 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.

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