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

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Message-ID: <20201006152623.sjc3jxagj4wh7g5f@gilmour.lan>
Date:   Tue, 6 Oct 2020 17:26:23 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Dave Stevenson <dave.stevenson@...pberrypi.com>
Cc:     Tim Gover <tim.gover@...pberrypi.com>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Eric Anholt <eric@...olt.net>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI Development <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 Dave,

On Fri, Oct 02, 2020 at 04:57:05PM +0100, Dave Stevenson wrote:
> Hi Maxime
> 
> On Fri, 2 Oct 2020 at 16:19, Maxime Ripard <maxime@...no.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.

I had in mind something like if the clock driver can infer that somehow
through some the boundaries reported by the firmware maybe? IIRC,
hdmi_enable_4kp60 will already change the max frequency reported to
550MHz instead of 500MHz

Maxime

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