linux-kernel - [RFC PATCH 1/2] drm/panel-edp: Allow overriding the eDP EDID

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Message-Id: <20220720155445.RFC.1.I2999ac2d08643f0c2f3fe916cca86f8c832c8142@changeid>
Date:   Wed, 20 Jul 2022 15:55:00 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     dri-devel@...ts.freedesktop.org
Cc:     ville.syrjala@...ux.intel.com, sam@...nborg.org,
        robdclark@...il.com, linus.walleij@...aro.org,
        thierry.reding@...il.com, bjorn.andersson@...aro.org,
        dmitry.baryshkov@...aro.org,
        Douglas Anderson <dianders@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Sean Paul <seanpaul@...omium.org>, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 1/2] drm/panel-edp: Allow overriding the eDP EDID

I found that writing to `/sys/kernel/debug/dri/*/eDP*/edid_override`
wasn't working for me. I could see the new EDID take effect in
`/sys/class/drm/card*-eDP*/edid` but userspace wasn't seeing it..

The problem was that panel-edp was caching the EDID that it read and
using that over and over again.

Let's change panel-edp to look at the EDID that's stored in the
connector. This is still a cache, which is important since this
function is called multiple times and readin the EDID is slow, but
this property is automatically updated by drm_get_edid() (which reads
the EDID) and also updated when writing the edid_override in debugfs.

Fixes: 63358e24ee79 ("drm/panel: panel-simple: Cache the EDID as long as we retain power")
Signed-off-by: Douglas Anderson <dianders@...omium.org>
---

 drivers/gpu/drm/panel/panel-edp.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/panel/panel-edp.c b/drivers/gpu/drm/panel/panel-edp.c
index 3626469c4cc2..12241c1e32f7 100644
--- a/drivers/gpu/drm/panel/panel-edp.c
+++ b/drivers/gpu/drm/panel/panel-edp.c
@@ -226,8 +226,6 @@ struct panel_edp {
 
 	const struct edp_panel_entry *detected_panel;
 
-	struct edid *edid;
-
 	struct drm_display_mode override_mode;
 
 	enum drm_panel_orientation orientation;
@@ -580,11 +578,19 @@ static int panel_edp_get_modes(struct drm_panel *panel,
 	if (p->ddc) {
 		pm_runtime_get_sync(panel->dev);
 
-		if (!p->edid)
-			p->edid = drm_get_edid(connector, p->ddc);
+		if (!connector->edid_blob_ptr) {
+			/*
+			 * We read the EDID and then immediately free it,
+			 * relying on the side effect of drm_get_edid() to store
+			 * a copy in connector->edid_blob_ptr. We always use
+			 * the copy cached in the connector since that allows
+			 * the edid_override to work.
+			 */
+			kfree(drm_get_edid(connector, p->ddc));
+		}
 
-		if (p->edid)
-			num += drm_add_edid_modes(connector, p->edid);
+		if (connector->edid_blob_ptr)
+			num += drm_add_edid_modes(connector, connector->edid_blob_ptr->data);
 
 		pm_runtime_mark_last_busy(panel->dev);
 		pm_runtime_put_autosuspend(panel->dev);
@@ -926,9 +932,6 @@ static int panel_edp_remove(struct device *dev)
 	if (panel->ddc && (!panel->aux || panel->ddc != &panel->aux->ddc))
 		put_device(&panel->ddc->dev);
 
-	kfree(panel->edid);
-	panel->edid = NULL;
-
 	return 0;
 }
 
-- 
2.37.0.170.g444d1eabd0-goog