linux-kernel - Re: [PATCH v3 02/10] drm/panel: Check for already prepared/enabled in drm

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Message-ID: <i3cbgrc365lwscwux2itho6uv74ji3hsjuge4zoxfnlnhacyqc@r73mmifyxffj>
Date:   Thu, 27 Jul 2023 08:37:56 +0200
From:   Maxime Ripard <mripard@...nel.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        cros-qcom-dts-watchers@...omium.org,
        Chris Morgan <macroalpha82@...il.com>,
        linux-input@...r.kernel.org, hsinyi@...gle.com,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        devicetree@...r.kernel.org, Daniel Vetter <daniel@...ll.ch>,
        yangcong5@...qin.corp-partner.google.com
Subject: Re: [PATCH v3 02/10] drm/panel: Check for already prepared/enabled
 in drm_panel

Hi,

On Wed, Jul 26, 2023 at 08:10:33AM -0700, Doug Anderson wrote:
> On Wed, Jul 26, 2023 at 5:41 AM Maxime Ripard <mripard@...nel.org> wrote:
> > On Tue, Jul 25, 2023 at 01:34:37PM -0700, Douglas Anderson wrote:
> > > NOTE: arguably, the right thing to do here is actually to skip this
> > > patch and simply remove all the extra checks from the individual
> > > drivers. Perhaps the checks were needed at some point in time in the
> > > past but maybe they no longer are? Certainly as we continue
> > > transitioning over to "panel_bridge" then we expect there to be much
> > > less variety in how these calls are made. When we're called as part of
> > > the bridge chain, things should be pretty simple. In fact, there was
> > > some discussion in the past about these checks [1], including a
> > > discussion about whether the checks were needed and whether the calls
> > > ought to be refcounted. At the time, I decided not to mess with it
> > > because it felt too risky.
> >
> > Yeah, I'd agree here too. I've never found evidence that it was actually
> > needed and it really looks like cargo cult to me.
> >
> > And if it was needed, then I'm not sure we need refcounting either. We
> > don't have refcounting for atomic_enable / disable, we have a sound API
> > design that makes sure we don't fall into that trap :)
> >
> > > Looking closer at it now, I'm fairly certain that nothing in the
> > > existing codebase is expecting these calls to be refcounted. The only
> > > real question is whether someone is already doing something to ensure
> > > prepare()/unprepare() match and enabled()/disable() match. I would say
> > > that, even if there is something else ensuring that things match,
> > > there's enough complexity that adding an extra bool and an extra
> > > double-check here is a good idea. Let's add a drm_warn() to let people
> > > know that it's considered a minor error to take advantage of
> > > drm_panel's double-checking but we'll still make things work fine.
> >
> > I'm ok with this, if we follow-up in a couple of releases and remove it
> > and all the calls.
> >
> > Could you add a TODO item so that we can keep a track of it? A follow-up
> > is fine if you don't send a new version of that series.
> 
> By this, I think you mean to add a "TODO" comment inline in the code?

No, sorry, I meant an entry in our TODO list: Documentation/gpu/todo.rst

> Also: I was thinking that we'd keep the check in "drm_panel.c" with
> the warning message indefinitely. You think it should be eventually
> removed? If we are truly thinking of removing it eventually, this
> feels like it should be a more serious warning message like a WARN(1,
> ...) to make it really obvious to people that they're relying on
> behavior that will eventually go away.

Yeah, it really feels like this is cargo-cult to me. Your approach seems
like a good short-term thing to do to warn everyone but eventually we'll
want it to go away.

So promoting it to a WARN could be a good thing, or let's say we do a
drm_warn for now, WARN next release, and gone in two?

Maxime

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