linux-kernel - Re: [PATCH] drm/panel-edp: use put

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Date: Mon, 8 Jan 2024 13:34:21 -0800
From: Doug Anderson <dianders@...omium.org>
To: Hsin-Yi Wang <hsinyi@...omium.org>
Cc: Neil Armstrong <neil.armstrong@...aro.org>, Jessica Zhang <quic_jesszhan@...cinc.com>, 
	Sam Ravnborg <sam@...nborg.org>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Linus Walleij <linus.walleij@...aro.org>, dri-devel@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/panel-edp: use put_sync in unprepare

Hi,

On Wed, Dec 20, 2023 at 2:43 PM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Wed, Dec 20, 2023 at 2:14 PM Hsin-Yi Wang <hsinyi@...omium.org> wrote:
> >
> > Some edp panel requires T10 (Delay from end of valid video data transmitted
> > by the Source device to power-off) less than 500ms. Using autosuspend with
> > delay set as 1000 violates this requirement.
> >
> > Use put_sync_suspend in unprepare to meet the spec. For other cases (such
> > as getting EDID), it still uses autosuspend.
> >
> > Suggested-by: Douglas Anderson <dianders@...omium.org>
> > Fixes: 5f04e7ce392d ("drm/panel-edp: Split eDP panels out of panel-simple")
>
> Probably instead:
>
> Fixes: 3235b0f20a0a ("drm/panel: panel-simple: Use runtime pm to avoid
> excessive unprepare / prepare")
>
> ...you could send a new version or I could just fix it up when I apply it.
>
>
> > Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
> > ---
> >  drivers/gpu/drm/panel/panel-edp.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
>
> Yeah, it's really unfortunate but I think we have to do this. It will
> add big delays any time we need to turn the panel off and quickly back
> on again, but I don't think we can reliably meet T10 without it. Even
> turning down the autosuspend delay won't really help since someone
> could do something like read the EDID while the delay was happening to
> reset the delay. At least we can still use "autosuspend" to avoid
> powering off between reading the EDID and powering up the panel since
> the EDID grabs runtime_pm itself and still uses autosuspend.
>
> I don't remember this particular problem before and nobody has yelled
> about it in the past. ...and the requirement seems crazy, but it
> certainly is in the spec sheets so we should be good citizens and
> honor it. On the plus side, this means that we will always fully power
> cycle the panel whenever we turn video off and that means that if any
> other panels out there have weird issues like "samsung-atna33xc20"
> this will also fix them since this is the same fix I had to do in that
> driver.
>
> In any case:
>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>
>
> I'm about to go on vacation, so I won't apply this until January.
> Other drm-misc maintainers are free to apply sooner than that if
> they're comfortable with it.

Things were silent and I'm back from vacation, so I've gone ahead and
pushed this to drm-misc-next.

Technically I could have pushed it to drm-misc-fixes, but from my
understanding of the issue it was not causing any actual problems
other than making someone upset who was staring at oscilloscope traces
and comparing them to a spec sheet. Given that this changes timings,
I'd rather have the extra bake time of going through drm-misc-next.

49ddab089611 drm/panel-edp: use put_sync in unprepare


-Doug