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

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Message-ID: <20201002151954.wazqc5riesdomlpx@gilmour.lan>
Date:   Fri, 2 Oct 2020 17:19:54 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Tim Gover <tim.gover@...pberrypi.com>
Cc:     Stefan Wahren <stefan.wahren@...e.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Eric Anholt <eric@...olt.net>,
        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 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)

Thanks!
Maxime

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