Re: [PATCH v5 17/20] drm/panel: panel-simple: Power the panel when reading the EDID

From: Sean Paul <seanpaul@chromium.org>
To: Douglas Anderson <dianders@chromium.org>
Cc: 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>,
	robdclark@chromium.org,
	dri-devel <dri-devel@lists.freedesktop.org>,
	David Airlie <airlied@linux.ie>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Steev Klimaszewski <steev@kali.org>,
	Stephen Boyd <swboyd@chromium.org>,
	Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	linux-i2c@vger.kernel.org,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5 17/20] drm/panel: panel-simple: Power the panel when reading the EDID
Date: Wed, 28 Apr 2021 12:44:45 -0400	[thread overview]
Message-ID: <CAOw6vbLqDgqhXH8pUZqfhKCZk68SfzLf76Z4au3g83=Uw_8z2Q@mail.gmail.com> (raw)
In-Reply-To: <20210423095743.v5.17.Ibd31b8f7c73255d68c5c9f5b611b4bfaa036f727@changeid>

On Fri, Apr 23, 2021 at 1:00 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> I don't believe that it ever makes sense to read the EDID when a panel
> is not powered and the powering on of the panel is the job of
> prepare(). Let's make sure that this happens before we try to read the
> EDID. We use the pm_runtime functions directly rather than directly
> calling the normal prepare() function because the pm_runtime functions
> are definitely refcounted whereas it's less clear if the prepare() one
> is.
>
> NOTE: I'm not 100% sure how EDID reading was working for folks in the
> past, but I can only assume that it was failing on the initial attempt
> and then working only later. This patch, presumably, will fix that. If
> some panel out there really can read the EDID without powering up and
> it's a big advantage to preserve the old behavior we can add a
> per-panel flag. It appears that providing the DDC bus to the panel in
> the past was somewhat uncommon in any case.
>

Maybe some combination of drivers caching the EDID for panels while
they're already powered and overly broad pm runtime references?

At any rate, this makes sense to me,

Reviewed-by: Sean Paul <seanpaul@chromium.org>

> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
>
> (no changes since v1)
>
>  drivers/gpu/drm/panel/panel-simple.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> index 4de33c929a59..a12dfe8b8d90 100644
> --- a/drivers/gpu/drm/panel/panel-simple.c
> +++ b/drivers/gpu/drm/panel/panel-simple.c
> @@ -510,12 +510,18 @@ static int panel_simple_get_modes(struct drm_panel *panel,
>
>         /* probe EDID if a DDC bus is available */
>         if (p->ddc) {
> -               struct edid *edid = drm_get_edid(connector, p->ddc);
> +               struct edid *edid;
>
> +               pm_runtime_get_sync(panel->dev);
> +
> +               edid = drm_get_edid(connector, p->ddc);
>                 if (edid) {
>                         num += drm_add_edid_modes(connector, edid);
>                         kfree(edid);
>                 }
> +
> +               pm_runtime_mark_last_busy(panel->dev);
> +               pm_runtime_put_autosuspend(panel->dev);
>         }
>
>         /* add hard-coded panel modes */
> --
> 2.31.1.498.g6c1eba8ee3d-goog
>
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