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

[Stefan Wahren]

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@xxxxxxx>
> > > >
> > > > 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