linux-kernel - Re: [PATCH v5 80/80] ARM: dts: bcm2711: Enable the display pipeline

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Date:   Thu, 1 Oct 2020 08:48:43 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Eric Anholt <eric@...olt.net>,
        Tim Gover <tim.gover@...pberrypi.com>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Hoegeun Kwon <hoegeun.kwon@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org,
        Phil Elwell <phil@...pberrypi.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5 80/80] ARM: dts: bcm2711: Enable the display pipeline

Hi Stefan,

On Wed, Sep 30, 2020 at 06:52:13PM +0200, Stefan Wahren wrote:
> Am 30.09.20 um 18:38 schrieb Nathan Chancellor:
> > On Wed, Sep 30, 2020 at 04:07:58PM +0200, Maxime Ripard wrote:
> >> Hi Nathan,
> >>
> >> On Tue, Sep 29, 2020 at 03:15:26PM -0700, Nathan Chancellor wrote:
> >>> On Thu, Sep 03, 2020 at 10:01:52AM +0200, Maxime Ripard wrote:
> >>>> Now that all the drivers have been adjusted for it, let's bring in the
> >>>> necessary device tree changes.
> >>>>
> >>>> The VEC and PV3 are left out for now, since it will require a more specific
> >>>> clock setup.
> >>>>
> >>>> Reviewed-by: Dave Stevenson <dave.stevenson@...pberrypi.com>
> >>>> Tested-by: Chanwoo Choi <cw00.choi@...sung.com>
> >>>> Tested-by: Hoegeun Kwon <hoegeun.kwon@...sung.com>
> >>>> Tested-by: Stefan Wahren <stefan.wahren@...e.com>
> >>>> Signed-off-by: Maxime Ripard <maxime@...no.tech>
> >>> Apologies if this has already been reported or have a solution but this
> >>> patch (and presumably series) breaks output to the serial console after
> >>> a certain point during init. On Raspbian, I see systemd startup messages
> >>> then the output just turns into complete garbage. It looks like this
> >>> patch is merged first in linux-next, which is why my bisect fell on the
> >>> DRM merge. I am happy to provide whatever information could be helpful
> >>> for debugging this. I am on the latest version of the firmware
> >>> (currently 26620cc9a63c6cb9965374d509479b4ee2c30241).
> >> Unfortunately, the miniUART is in the same clock tree than the core
> >> clock and will thus have those kind of issues when the core clock is
> >> changed (which is also something that one should expect when using the
> >> DRM or other drivers).
> >>
> >> The only real workaround there would be to switch to one of the PL011
> >> UARTs. I guess we can also somehow make the UART react to the core clock
> >> frequency changes, but that's going to require some effort
> >>
> >> Maxime
> > Ack, thank you for the reply! There does not really seem to be a whole
> > ton of documentation around using one of the other PL011 UARTs so for
> > now, I will just revert this commit locally.
> 
> there was a patch series & discussion about this topic, but we finally
> didn't find a rock solid solution.
> 
> You can have a look at "[RFC 5/5] serial: 8250: bcm2835aux: add notifier
> to follow clock changes" from 3.4.2019 on linux-rpi-kernel.

I couldn't find that discussion on the archive, but based on the title I
guess there's some patches that have been merged this cycle for the 8250
driver to do just that (868f3ee6e452 ("serial: 8250: Add 8250 port clock
update method") and cc816969d7b5 ("serial: 8250_dw: Fix common clocks
usage race condition")).

However, I'm not entirely sure the clock notifier works in our case with
the firmware / MMIO clocks duality

Maxime