linux-kernel - [PATCH v4 23/27] drm/panel: panel-simple: Power the panel when reading the EDID

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Message-Id: <20210416153909.v4.23.Ibd31b8f7c73255d68c5c9f5b611b4bfaa036f727@changeid>
Date:   Fri, 16 Apr 2021 15:39:46 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Sam Ravnborg <sam@...nborg.org>, Wolfram Sang <wsa@...nel.org>
Cc:     Stephen Boyd <swboyd@...omium.org>, robdclark@...omium.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Stanislav Lisovskiy <stanislav.lisovskiy@...el.com>,
        Steev Klimaszewski <steev@...i.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-msm@...r.kernel.org, Linus W <linus.walleij@...aro.org>,
        Douglas Anderson <dianders@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Thierry Reding <thierry.reding@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: [PATCH v4 23/27] drm/panel: panel-simple: Power the panel when reading the EDID

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.

Signed-off-by: Douglas Anderson <dianders@...omium.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 c91e8aa108f7..40382c1be692 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 */
-- 
2.31.1.368.gbe11c130af-goog