Re: [RFC 1/5] drm/i915/dp: Program source OUI on eDP panels

[Lyude Paul <lyude@redhat.com>]

On Tue, 2020-09-15 at 15:06 -0400, Rodrigo Vivi wrote:
> On Tue, Sep 15, 2020 at 01:29:35PM -0400, Lyude Paul wrote:
> > Since we're about to start adding support for Intel's magic HDR
> > backlight interface over DPCD, we need to ensure we're properly
> > programming this field so that Intel specific sink services are exposed.
> > Otherwise, 0x300-0x3ff will just read zeroes.
> > 
> > We also take care not to reprogram the source OUI if it already matches
> > what we expect. This is just to be careful so that we don't accidentally
> > take the panel out of any backlight control modes we found it in.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > Cc: thaytan@noraisin.net
> > Cc: Vasily Khoruzhick <anarsoul@gmail.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp.c | 32 +++++++++++++++++++++++++
> >  1 file changed, 32 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c
> > b/drivers/gpu/drm/i915/display/intel_dp.c
> > index 4bd10456ad188..b591672ec4eab 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > @@ -3428,6 +3428,7 @@ void intel_dp_sink_set_decompression_state(struct
> > intel_dp *intel_dp,
> >  void intel_dp_sink_dpms(struct intel_dp *intel_dp, int mode)
> >  {
> >  	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> > +	u8 edp_oui[] = { 0x00, 0xaa, 0x01 };
> 
> what are these values?

I wish I knew, my assumption is this is the OUI that Intel's GPU driver uses on
other platforms, but I don't have any documentation mentioning this (in fact,
the few documents I do have on this backlight interface don't seem to make any
mention of it). I only managed to find this when looking through the last
attempt someone did at adding support for this backlight interface:

https://patchwork.freedesktop.org/patch/334989/

I think it should be fairly safe to write, as I know nouveau always programs a
source OUI (we don't do it from our driver, but nvidia hardware seems to do it
automatically) and I don't believe I've seen it ever change any behavior besides
making things appear in the 0x300-0x3ff register range.

AFAICT though, the backlight interface won't advertise itself without this being
set early on. If you could find anyone from Intel who knows more about it though
I'd definitely appreciate it (and just in general for the rest of the patch
series as well)

> 
> >  	int ret, i;
> >  
> >  	/* Should have a valid DPCD by this point */
> > @@ -3443,6 +3444,14 @@ void intel_dp_sink_dpms(struct intel_dp *intel_dp,
> > int mode)
> >  	} else {
> >  		struct intel_lspcon *lspcon = dp_to_lspcon(intel_dp);
> >  
> > +		/* Write the source OUI as early as possible */
> > +		if (intel_dp_is_edp(intel_dp)) {
> > +			ret = drm_dp_dpcd_write(&intel_dp->aux, DP_SOURCE_OUI,
> > edp_oui,
> > +						sizeof(edp_oui));
> > +			if (ret < 0)
> > +				drm_err(&i915->drm, "Failed to write eDP source
> > OUI\n");
> > +		}
> > +
> >  		/*
> >  		 * When turning on, we need to retry for 1ms to give the sink
> >  		 * time to wake up.
> > @@ -4530,6 +4539,23 @@ static void intel_dp_get_dsc_sink_cap(struct intel_dp
> > *intel_dp)
> >  	}
> >  }
> >  
> > +static void
> > +intel_edp_init_source_oui(struct intel_dp *intel_dp)
> > +{
> > +	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> > +	u8 oui[] = { 0x00, 0xaa, 0x01 };
> > +	u8 buf[3] = { 0 };
> > +
> > +	if (drm_dp_dpcd_read(&intel_dp->aux, DP_SOURCE_OUI, buf, sizeof(buf)) <
> > 0)
> > +		drm_err(&i915->drm, "Failed to read source OUI\n");
> > +
> > +	if (memcmp(oui, buf, sizeof(oui)) == 0)
> > +		return;
> > +
> > +	if (drm_dp_dpcd_write(&intel_dp->aux, DP_SOURCE_OUI, oui, sizeof(oui)) <
> > 0)
> > +		drm_err(&i915->drm, "Failed to write source OUI\n");
> > +}
> > +
> >  static bool
> >  intel_edp_init_dpcd(struct intel_dp *intel_dp)
> >  {
> > @@ -4607,6 +4633,12 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
> >  	if (INTEL_GEN(dev_priv) >= 10 || IS_GEMINILAKE(dev_priv))
> >  		intel_dp_get_dsc_sink_cap(intel_dp);
> >  
> > +	/*
> > +	 * Program our source OUI so we can make various Intel-specific AUX
> > +	 * services available (such as HDR backlight controls)
> > +	 */
> > +	intel_edp_init_source_oui(intel_dp);
> 
> I believe we should restrict this to the supported platforms: cfl, whl, cml,
> icl, tgl
> no?
> 
> > +
> >  	return true;
> >  }
> >  
> > -- 
> > 2.26.2
> > 
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
-- 
Sincerely,
      Lyude Paul (she/her)
      Software Engineer at Red Hat