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From:   Dave Stevenson <dave.stevenson@raspberrypi.com>
Date:   Fri, 2 Oct 2020 16:57:05 +0100
Message-ID: <CAPY8ntCkY9F0e=hOyg=rs5G2a=iEbukWgmr0adXrwJQPm=uY6A@mail.gmail.com>
Subject: Re: [PATCH v5 80/80] ARM: dts: bcm2711: Enable the display pipeline
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Tim Gover <tim.gover@raspberrypi.com>,
        Stefan Wahren <stefan.wahren@i2se.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@suse.de>,
        Eric Anholt <eric@anholt.net>,
        LKML <linux-kernel@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Hoegeun Kwon <hoegeun.kwon@samsung.com>,
        Chanwoo Choi <cw00.choi@samsung.com>,
        bcm-kernel-feedback-list@broadcom.com,
        linux-rpi-kernel@lists.infradead.org,
        Phil Elwell <phil@raspberrypi.com>,
        linux-arm-kernel@lists.infradead.org
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Hi Maxime

On Fri, 2 Oct 2020 at 16:19, Maxime Ripard <maxime@cerno.tech> wrote:
>
> Hi Tim,
>
> On Thu, Oct 01, 2020 at 11:15:46AM +0100, Tim Gover wrote:
> > hdmi_enable_4k60=1 causes the firmware to select 3.3 GHz for the PLLC
> > VCO to support a core-frequency of 550 MHz which is the minimum
> > frequency required by the HVS at 4Kp60. The side effect is that if the
> > display clock requirements are lower than 4Kp60 then you will see
> > different core frequencies selected by DVFS.
> >
> > If enable_uart=1 and the mini-uart is selected (default unless
> > bluetooth is disabled) then the firmware will pin the core-frequency
> > to either core_freq max (500 or 550). Although, I think there is a way
> > of pinning it to a lower fixed frequency.
> >
> > The table in overclocking.md defines options for setting the maximum
> > core frequency but unless core_freq_min is specified DVFS will
> > automatically pick the lowest idle frequency required by the display
> > resolution.
>
> I'm wondering if there's some way to detect this from Linux? I guess it
> would be nice to be able to at least detect a broken config to warn /
> prevent an user that their situation is not going to be reliable / work
> really well (like if they have a 4k display without hdmi_enable_4kp60
> set, or the issue we're discussing here)

The main filter in the firmware is the parameter
hdmi_pixel_freq_limit. That can either be set manually from
config.txt, or defaults appropriately based on hdmi_enable_4kp60.
Under firmware_kms [1] I read back those values to use as a filter
within crtc_mode_valid[2].
I can't think of a nice way of exposing that without the vc4 driver
gaining a DT link to the firmware, and that starts to get ugly.

  Dave

[1] https://github.com/raspberrypi/linux/blob/rpi-5.9.y/drivers/gpu/drm/vc4/vc4_firmware_kms.c#L1859
[2] https://github.com/raspberrypi/linux/blob/rpi-5.9.y/drivers/gpu/drm/vc4/vc4_firmware_kms.c#L1077