[PATCH v2] drm/probe-helper: Default to 640x480 if no EDID

[Douglas Anderson <dianders@chromium.org>]

If we're unable to read the EDID for a display because it's corrupt /
bogus / invalid then we'll add a set of standard modes for the
display. When userspace looks at these modes it doesn't really have a
good concept for which mode to pick and it'll likely pick the highest
resolution one by default. That's probably not ideal because the modes
were purely guesses on the part of the Linux kernel.

Let's instead set 640x480 as the "preferred" mode when we have no EDID.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---
Note that this is the second of two related and similar-sounding but
different patches. See also ("drm/probe-helper: For DP, add 640x480 if
all other modes are bad") [1]. I'm hoping to land _both_ of the
patches since they address different issues. This patch addresses the
case of a corrupt EDID and having 640x480 be the default in the
"guessed" modes. The other patch handles the case where the EDID
_isn't_ corrupt but all the modes listed can't be made with the
existing situations. The two patches can land in either order.

Also note that I didn't carry any Tested-by / Reviewed-by tags since
the patch is now quite different.

[1] https://lore.kernel.org/r/20220510131309.v2.2.I4ac7f55aa446699f8c200a23c10463256f6f439f@changeid

Changes in v2:
- Don't modify drm_add_modes_noedid() 'cause that'll break others
- Set 640x480 as preferred in drm_helper_probe_single_connector_modes()

 drivers/gpu/drm/drm_probe_helper.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
index 682359512996..1fbb9a8c315c 100644
--- a/drivers/gpu/drm/drm_probe_helper.c
+++ b/drivers/gpu/drm/drm_probe_helper.c
@@ -516,8 +516,17 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
 		count = drm_add_override_edid_modes(connector);
 
 	if (count == 0 && (connector->status == connector_status_connected ||
-			   connector->status == connector_status_unknown))
+			   connector->status == connector_status_unknown)) {
 		count = drm_add_modes_noedid(connector, 1024, 768);
+
+		/*
+		 * Give userspace a hint that we don't have a lot of confidence
+		 * in these modes (we totally guessed) by marking 640x480 as
+		 * preferred. This is low clock rate and incredibly common as
+		 * a failsafe mode.
+		 */
+		drm_set_preferred_mode(connector, 640, 480);
+	}
 	count += drm_helper_probe_add_cmdline_mode(connector);
 	if (count == 0)
 		goto prune;
-- 
2.36.0.550.gb090851708-goog