linux-kernel - [PATCH] drm/edid: Don't print an error if the checksum of a CEA block is wrong

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Message-Id: <1481202716-31317-1-git-send-email-tomeu.vizoso@collabora.com>
Date:   Thu,  8 Dec 2016 14:11:56 +0100
From:   Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:     linux-kernel@...r.kernel.org
Cc:     Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Tomi Sarvela <tomi.p.sarvela@...el.com>,
        intel-gfx@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org
Subject: [PATCH] drm/edid: Don't print an error if the checksum of a CEA block is wrong

It's common to share screens within CI labs, and it's also common for
KVM switches to alter the contents of the CEA block but leave the
checksum outdated.

So in this case, print a debug message instead of an error.

References: https://bugs.freedesktop.org/show_bug.cgi?id=98228
Cc: Chris Wilson <chris@...is-wilson.co.uk>
Cc: Tomi Sarvela <tomi.p.sarvela@...el.com>
Cc: intel-gfx@...ts.freedesktop.org
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@...labora.com>
---
 drivers/gpu/drm/drm_edid.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 6798c3ad9d53..db79bc949216 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -1128,16 +1128,19 @@ bool drm_edid_block_valid(u8 *raw_edid, int block, bool print_bad_edid,
 
 	csum = drm_edid_block_checksum(raw_edid);
 	if (csum) {
-		if (print_bad_edid) {
-			DRM_ERROR("EDID checksum is invalid, remainder is %d\n", csum);
-		}
-
 		if (edid_corrupt)
 			*edid_corrupt = true;
 
 		/* allow CEA to slide through, switches mangle this */
-		if (raw_edid[0] != 0x02)
+		if (raw_edid[0] == CEA_EXT) {
+			DRM_DEBUG("EDID checksum is invalid, remainder is %d\n", csum);
+			DRM_DEBUG("Assuming a KVM switch modified the CEA block but left the original checksum\n");
+		} else {
+			if (print_bad_edid)
+				DRM_ERROR("EDID checksum is invalid, remainder is %d\n", csum);
+
 			goto bad;
+		}
 	}
 
 	/* per-block-type checks */
-- 
2.7.4