[PATCH v3] drm: Only create a cmdline mode if no probed modes match

[Chris Wilson]

The intention of using video=<connector>:<mode> is primarily to select
the user's preferred resolution at startup. Currently we always create a
new mode irrespective of whether the monitor has a native mode at the
desired resolution. This has the issue that we may then select the fake
mode rather the native mode during fb_helper->inital_config() and so
if the fake mode is invalid we then end up with a loss of signal. Oops.
This invalid fake mode would also be exported to userspace, who
potentially may make the same mistake.

To avoid this issue, we filter out the added command line mode if we
detect the desired resolution (and clock if specified) amongst the
probed modes. This fixes the immediate problem of adding a duplicate
mode, but perhaps more generically we should avoid adding a GTF mode if
the monitor has an EDID that is not GTF-compatible, or similarly for
CVT.

Fixes regression from

commit eaf99c749d43ae74ac7ffece5512f3c73f01dfd2
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Wed Aug 6 10:08:32 2014 +0200

    drm: Perform cmdline mode parsing during connector initialisation

that breaks HDMI output on BeagleBone Black with LG TV (model 19LS4R-ZA).

v2: Explicitly delete our earlier cmdline mode
v3: Mode pruning should now be sufficient to delete stale cmdline modes

Reported-by: Radek Dostál <rd at radekdostal.com>
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Radek Dostál <rd at radekdostal.com>
Cc: Jesse Barnes <jbarnes at virtuousgeek.org>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: dri-devel at lists.freedesktop.org
Cc: Julia Lemire <jlemire at matrox.com>
Cc: Dave Airlie <airlied at redhat.com>
Cc: stable at vger.kernel.org
---
 drivers/gpu/drm/drm_probe_helper.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
index 0329080d7f7c..a705ed12c062 100644
--- a/drivers/gpu/drm/drm_probe_helper.c
+++ b/drivers/gpu/drm/drm_probe_helper.c
@@ -82,13 +82,28 @@ drm_mode_validate_flag(const struct drm_display_mode *mode,
 
 static int drm_helper_probe_add_cmdline_mode(struct drm_connector *connector)
 {
+	struct drm_cmdline_mode *cmdline_mode;
 	struct drm_display_mode *mode;
 
-	if (!connector->cmdline_mode.specified)
+	cmdline_mode = &connector->cmdline_mode;
+	if (!cmdline_mode->specified)
 		return 0;
 
+	/* Only add a GTF mode if we find no matching probed modes */
+	list_for_each_entry(mode, &connector->probed_modes, head) {
+		if (mode->hdisplay != cmdline_mode->xres ||
+		    mode->vdisplay != cmdline_mode->yres)
+			continue;
+
+		if (cmdline_mode->refresh_specified &&
+		    mode->vrefresh != cmdline_mode->refresh)
+			continue;
+
+		return 0;
+	}
+
 	mode = drm_mode_create_from_cmdline_mode(connector->dev,
-						 &connector->cmdline_mode);
+						 cmdline_mode);
 	if (mode == NULL)
 		return 0;
 
-- 
2.8.1