Re: [PATCH] drm/bridge: analogix_dp: Make PSR-disable non-blocking

[Brian Norris <briannorris@chromium.org>]

On Wed, Oct 20, 2021 at 06:23:35PM -0700, Brian Norris wrote:
> On Wed, Oct 20, 2021 at 5:40 PM Sean Paul <sean@poorly.run> wrote:
> > The actual latency gains from doing this synchronously are minimal since the
> > panel will display new content as soon as it can regardless of whether the
> > kernel is blocking. There is likely a perceptual difference, but that's only
> > because kernel is lying to userspace and skipping frames without consent.
> 
> Hmm, you might well be right about some of the first points (I'm still
> learning the DRM framework), but I'm a bit skeptical that the
> perceptual difference is "only" because we're cheating in some way.
> I'm not doing science here, and it's certainly not a blinded test, but
> I'm nearly certain this patch cuts out approx 50-80% of the cursor lag
> I see without this patch (relative to the current Chrome OS kernel). I
> don't see how cheating would produce a smoother cursor movement --
> we'd still be dropping frames, and the movement would appear jumpy
> somewhere along the way.

Aha, so I think I found {a,the} reason for some disagreement here:
looking at the eDP PSR spec, I see that while the current implementation
is looking for psr_state==DP_PSR_SINK_INACTIVE to signal PSR-exit
completion, the spec shows an intermediate state
(DP_PSR_SINK_ACTIVE_RESYNC == 4), where among other things, "the Sink
device must display the incoming active frames from the Source device
with no visible glitches and/or artifacts."

And it happens that we move to DP_PSR_SINK_ACTIVE_RESYNC somewhat
quickly (on the order of 20-40ms), while the move to
DP_PSR_SINK_INACTIVE is a good chunk longer (approx 60ms more). So
pre-commit-6c836d965bad might have been cheating a little (we'd claim
we're "done" about 20-40ms too early), but post-commit-6c836d965bad
we're waiting about 60ms too long.

I'll send v2 to make this block for DP_PSR_SINK_ACTIVE_RESYNC ||
DP_PSR_SINK_INACTIVE, which gets much or all of the same latency win,
and I'll try to document the reasons, etc., better.

I'll probably also include a patch to drop the 'blocking' parameter,
since it's unused, and gives the wrong idea about this state machine.

> In any case, I'm absolutely certain that mainline Linux performs much
> much worse with PSR than the current CrOS kernel, but there are some
> other potential reasons for that, such as the lack of an
> input-notifier [1].
...
> [1] This got locked up in "controversy":
> https://patchwork.kernel.org/project/linux-arm-kernel/patch/20180405095000.9756-25-enric.balletbo@collabora.com/

While I'm here: I also played with this a bit, and I still haven't
gotten all the details right, but I don't believe this alone will get
the latency wins we'd like. We still need something like the above.

Brian