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

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Message-ID: <CAD=FV=VdO=V51jbK5Ym9g9m2naJ4tfC-vKFCnfbpT+F-pNM_OA@mail.gmail.com>
Date: Wed, 20 Dec 2023 14:43:02 -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 2:14 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.