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

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Message-ID: <snx3fzvf3icauri2xuigydvpqxtzhp34mptdxvifi7jswm2evy@sx7jr7zwvjw5>
Date:   Wed, 26 Jul 2023 14:41:51 +0200
From:   Maxime Ripard <mripard@...nel.org>
To:     Douglas 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 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.

Maxime

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