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From:   Dave Stevenson <dave.stevenson@raspberrypi.com>
Date:   Tue, 6 Oct 2020 18:14:01 +0100
Message-ID: <CAPY8ntAr+wV0F-GCxHLg+30tuu8van51BFQ9Nh_C1LD+CPRcLA@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 Tue, 6 Oct 2020 at 16:26, Maxime Ripard <maxime@cerno.tech> wrote:
>
> 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@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.
>
> 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

Yes, that's plausible, but I don't know enough about the clock
infrastructure for advertising limits to know what works there.
Tell me what you need from the mailbox service and I'll see what I can do.

We do already have RPI_FIRMWARE_GET_MAX_CLOCK_RATE and
RPI_FIRMWARE_GET_MIN_CLOCK_RATE. It'd take a few minutes of staring at
the code (or a quick test) to confirm if they definitely are changed
for CORE clock by hdmi_enable_4kp60 - I think it does.

  Dave