Re: [PATCH] drm/vc4: Fix HDMI mode validation

[Nicolas Saenz Julienne]

On Thu, 2020-03-26 at 18:19 +0100, Stefan Wahren wrote:
> Am 26.03.20 um 13:20 schrieb Nicolas Saenz Julienne:
> > Current mode validation impedes setting up some video modes which should
> > be supported otherwise. Namely 1920x1200@60Hz.
> > 
> > Fix this by lowering the minimum HDMI state machine clock to pixel clock
> > ratio allowed.
> > 
> > Fixes: 32e823c63e90 ("drm/vc4: Reject HDMI modes with too high of clocks")
> > Reported-by: Stefan Wahren <stefan.wahren@xxxxxxxx>
> > Suggested-by: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
> > ---
> >  drivers/gpu/drm/vc4/vc4_hdmi.c | 20 ++++++++++++++++----
> >  1 file changed, 16 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > index cea18dc15f77..340719238753 100644
> > --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> > +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > @@ -681,11 +681,23 @@ static enum drm_mode_status
> >  vc4_hdmi_encoder_mode_valid(struct drm_encoder *crtc,
> >  			    const struct drm_display_mode *mode)
> >  {
> > -	/* HSM clock must be 108% of the pixel clock.  Additionally,
> > -	 * the AXI clock needs to be at least 25% of pixel clock, but
> > -	 * HSM ends up being the limiting factor.
> > +	/*
> > +	 * As stated in RPi's vc4 firmware "HDMI state machine (HSM) clock must
> > +	 * be faster than pixel clock, infinitesimally faster, tested in
> > +	 * simulation. Otherwise, exact value is unimportant for HDMI
> > +	 * operation." This conflicts with bcm2835's vc4 documentation, which
> > +	 * states HSM's clock has to be at least 108% of the pixel clock.
> > +	 *
> > +	 * Real life tests reveal that vc4's firmware statement holds up, and
> > +	 * users are able to use pixel clocks closer to HSM's, namely for
> > +	 * 1920x1200@60Hz. So it was decided to have leave a 1% margin between
> > +	 * both clocks. Which, for RPi0-3 implies a maximum pixel clock of
> 
> s/RPi0-3/bcm2835/ ?

Well as Dave Stevenson stated on the previous thread[1], it's the firmware that
sets up the HSM limitation. IIUC technically both HSM and pixel clocks could be
increased. Hence this being more of a RPi limitation than the chip itself.

"Whilst the firmware would appear to use a fixed HSM clock on Pi0-3, I
don't anticipate there being any issue with varying it. It looks like
there was the expectation of it being variable in the past, but
someone has refactored it and either accidentally or deliberately
given up on the idea."

Regards,
Nicolas

[1] https://www.spinics.net/lists/arm-kernel/msg794591.html

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