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

Maxime