Re: [PATCH] drm/i915/dp: Add current maximum eDP link rate to sink_rate array.

From: Mario Kleiner <mario.kleiner.de@gmail.com>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: mario.kleiner.de@gmail.de,
	intel-gfx <intel-gfx@lists.freedesktop.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Daniel Vetter <daniel.vetter@ffwll.ch>
Subject: Re: [PATCH] drm/i915/dp: Add current maximum eDP link rate to sink_rate array.
Date: Thu, 9 Jan 2020 18:57:14 +0100	[thread overview]
Message-ID: <CAEsyxyhjbP6ADutU7XRJUjryj1+X8mFqopB9TvBoW6RWjBihww@mail.gmail.com> (raw)
In-Reply-To: <20200109164715.GD13686@intel.com>

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On Thu, Jan 9, 2020 at 5:47 PM Ville Syrjälä <ville.syrjala@linux.intel.com>
wrote:

> On Thu, Jan 09, 2020 at 05:30:05PM +0100, Mario Kleiner wrote:
> > On Thu, Jan 9, 2020 at 4:38 PM Ville Syrjälä <
> ville.syrjala@linux.intel.com>
> > wrote:
> >
> > > On Thu, Jan 09, 2020 at 05:26:57PM +0200, Ville Syrjälä wrote:
> > > > On Thu, Jan 09, 2020 at 04:07:52PM +0100, Mario Kleiner wrote:
> > > > > The panel reports 10 bpc color depth in its EDID, and the UEFI
> > > > > firmware chooses link settings at boot which support enough
> > > > > bandwidth for 10 bpc (324000 kbit/sec to be precise), but the
> > > > > DP_MAX_LINK_RATE dpcd register only reports 2.7 Gbps as possible,
> > >
> > > Does it actually or do we just ignore the fact that it reports
> 3.24Gbps?
> > >
> > > If it really reports 3.24 then we should be able to just add that to
> > > dp_rates[] in intel_dp_set_sink_rates() and be done with it.
> > >
> > > Although we'd likely want to skip 3.24 unless it really is reported
> > > as the max so as to not use that non-standard rate on other displays.
> > > So would require a bit fancier logic for that.
> > >
> > >
> > Was also my initial thought, but the DP_MAX_LINK_RATE reg reports 2.7
> Gbps
> > as maximum.
>
> So dpcd[0x1] == 0xa ?
>
>
Yes. [*]

> What about the magic second version of DP_MAX_LINK_RATE at 0x2201 ?
> Hmm. I guess we should already be reading that via
> intel_dp_extended_receiver_capabilities().
>

Yes, you do.

[*] Well, i have to recheck on the machine. I started this work on the AMD
side and checked what AMD DC gave me, haven't rechecked stuff under i915
that i already knew from AMD. Comparing the implementations, there's some
peculiar differences that may matter:

intel_dp_extended_receiver_capabilities() is more "paranoid" than AMD DC's
retrieve_link_cap() function in deciding if the extended receiver caps are
valid. Intels implementation copies only the first 6 Bytes of extended
receiver caps into the dpcd[] arrays, whereas AMD copies 16 Bytes. Not sure
about the differences, but one of you may wanna check why this is, and if
it matters somehow.

Btw. your proposed

/* blah */
if (max_rate > ...)

wouldn't work if dpcd[0x1] == 0xa, which it likely is [*]. AMD DC
identified it as DP 1.1, eDP 1.3, and these extended caps seem to be only
part of DP 1.3+ if i understand the comments in
intel_dp_extended_receiver_capabilities() correctly.

-mario

>
> --
> Ville Syrjälä
> Intel
>

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