Re: [PATCH] arm64: dts: rockchip: Change serial baud rate for Pinephone Pro to 1.5 MB

[Javier Martinez Canillas]

Heiko Stübner <heiko@xxxxxxxxx> writes:

Hello Heiko,

> Hi Peter,
>
> Am Dienstag, 4. April 2023, 14:52:02 CEST schrieb Peter Geis:
>> On Tue, Apr 4, 2023 at 3:55 AM Heiko Stübner <heiko@xxxxxxxxx> wrote:
>> >
>> > Hi,
>> >
>> > Am Montag, 3. April 2023, 19:59:37 CEST schrieb Javier Martinez Canillas:
>> > > This baud rate is set for the device by mainline u-boot and is also what
>> > > is set in the Pinebook Pro Device Tree, which is a device similar to the
>> > > PinePhone Pro but with a different form factor.
>> > >
>> > > Otherwise, the baud rate of the firmware and Linux don't match by default
>> > > and a 'console=ttyS2,1500000n8' kernel command line parameter is required
>> > > to have proper output for both.
>> >
>> > The interesting question is always if this will break someone else's setup.
>> > I've never really understood the strange setting of 1.5MBps, but on the
>> > other hand it _is_ a reality on most boards.
>
>> The 1.5M baud is default because the clock structure on rockchip
>> devices does not allow a clean 115200 baud. By attempting to force
>> 115200, it will always be slightly off (either low or high depending
>> on how the driver decided to round). If this actually causes any
>> problems is the subject of much debate.
>
> thanks so much for this piece of clock-detail. As I wrote, I never really
> understood the why _before_ but also never cared that much to dive
> into it and find out.
>
> So your explanation closes one knowledge gap in my head.
>
> Thanks a lot :-)

Did you make a decision about this? I guess the clock explanation is yet
another argument in favour of switching the PPP to a 1.5 Mbps baud rate ?

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat