Re: [PATCH v4 27/27] drm/panel: panel-simple: Prepare/unprepare are refcounted, not forced

From: Doug Anderson <dianders@chromium.org>
To: Andrzej Hajda <a.hajda@samsung.com>,
	Neil Armstrong <narmstrong@baylibre.com>,
	Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
	Jonas Karlman <jonas@kwiboo.se>,
	Jernej Skrabec <jernej.skrabec@siol.net>,
	Sam Ravnborg <sam@ravnborg.org>, Wolfram Sang <wsa@kernel.org>
Cc: Stephen Boyd <swboyd@chromium.org>,
	Rob Clark <robdclark@chromium.org>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>,
	Steev Klimaszewski <steev@kali.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Linus W <linus.walleij@linaro.org>,
	Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@linux.ie>,
	Thierry Reding <thierry.reding@gmail.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 27/27] drm/panel: panel-simple: Prepare/unprepare are refcounted, not forced
Date: Fri, 23 Apr 2021 09:46:57 -0700	[thread overview]
Message-ID: <CAD=FV=XdAA4+U5=oW++6j84+=L88ViuKRpGkmgUShDYj0=SaoQ@mail.gmail.com> (raw)
In-Reply-To: <20210416153909.v4.27.I502f2a92ddd36c3d28d014dd75e170c2d405a0a5@changeid>

Hi,

On Fri, Apr 16, 2021 at 3:41 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> Historically simple-panel had code to make sure that if prepare() was
> called on an already-prepared panel that it was a no-op. Similarly if
> unprepare() was called on an already-unprepared panel it was also a
> no-op. Essentially it means that these functions always "forced" the
> value to be whatever the caller wanted it to be. You can see that the
> forcing behavior dates back at least as far as 2014 by looking at
> commit 613a633e7a56 ("drm/panel: simple: Add proper definition for
> prepare and unprepare").
>
> Apparently the code supporting the historical behavior may not be
> needed [1] and prepare() / unprepare() are supposed to be
> balanced. Let's try removing it and see if anyone breaks! If they do
> then we can have a debate about whether we should change that code or
> revert this patch. :-) If nobody breaks then we've nicely saved a few
> lines of code and some complexity.
>
> [1] https://lore.kernel.org/r/YHePsQgqOau1V5lD@pendragon.ideasonboard.com
>
> Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
> (no changes since v1)
>
>  drivers/gpu/drm/panel/panel-simple.c | 14 --------------
>  1 file changed, 14 deletions(-)

So it turns out that when looking at panel_simple_remove() I already
found a case that's not necessarily refcounting. There I see
drm_panel_unprepare() just simply hardcoded, but (as I understand it)
there's no reason to believe that the panel has been prepared at
remove() time. Yes, I could fix panel_simple_remove() but instead, for
now, I think I'm going to drop this patch from the series. If someone
wants to pick it up then I certainly won't object, but for now keeping
the way things are seems the best way to keep from getting shouted at.

-Doug