linux-kernel - Re: [PATCH -next] drm/bridge: Add missing clk_disable_unprepare() in analogix_dp

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Message-ID: <Yv2CPBD3Picg/gVe@google.com>
Date:   Wed, 17 Aug 2022 17:05:16 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Zhang Zekun <zhangzekun11@...wei.com>
Cc:     Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Doug Anderson <dianders@...omium.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        xuqiang36@...wei.com
Subject: Re: [PATCH -next] drm/bridge: Add missing clk_disable_unprepare() in
 analogix_dp_resume()

On Wed, Aug 17, 2022 at 01:34:13PM -0700, Brian Norris wrote:
> On Mon, Aug 15, 2022 at 11:46 PM Zhang Zekun <zhangzekun11@...wei.com> wrote:
> >
> > Add the missing clk_disable_unprepare() before return from
> > analogix_dp_resume() in the error handling case.
> >
> > Fixes: 211f276ed3d9 ("drm: bridge: analogix/dp: add panel prepare/unprepare in suspend/resume time")
> > Signed-off-by: Zhang Zekun <zhangzekun11@...wei.com>
> 
> Reviewed-by: Brian Norris <briannorris@...omium.org>

Hmm, actually I'm going to have to retract that now that I've given it
some more testing locally. I happen to have a system where I commonly
hit this error case, and I'm thinking commit 211f276ed3d9 is actually
wrong, and so we shouldn't be "fixing" its error handling -- we should
be reverting it.

In particular, drm_panel_prepare()/drm_panel_unprepare() are *not*
reference-counted APIs, and this is already managed by
analogix_dp_bridge_disable(), which is called by the core DRM helpers
during suspend. Thus, analogix_dp_suspend()/analogix_dp_resume() is
serving to be an unwanted *second* client trying to {un,}prepare the
panel.

The panel drivers tend to handle this OK to some extent, as they (e.g.,
panel-edp.c) guard against redundant operations, but *we* don't --
notice that analogix_dp_suspend() ignores drm_panel_unprepare() errors
for one.

Also, I don't believe device management really handles resume() failures
quite right; in the end, this patch ends up un-balancing the clk count
on the RK3399 Gru-Bob systems I'm testing.

(Side note: every other bridge driver seems to ignore
drm_panel_prepare() failures.)

It's possible this was correct (or at least, not terribly broken) back
when it was written, but then, the DRM core frameworks have evolved
since then. Today, I think we do not need to manage the panel state
directly in suspend()/resume().

All in all:

Nacked-by: Brian Norris <briannorris@...omium.org>
Tested-and-failed-by: Brian Norris <briannorris@...omium.org>

Now separately, I have to figure out why I'm hitting this error case in
the first place...