drm/radeon: workaround a hw bug on some radeon chipsets with all-0 EDIDs.

Some RS690 chipsets seem to end up with floating connectors, either
a DVI connector isn't actually populated, or an add-in HDMI card
is available but not installed. In this case we seem to get a NULL byte
response for each byte of the i2c transaction, so we detect this
case and if we see it we don't do anymore DDC transactions on this
connector.

I've tested this on my RS690 without the HDMI card installed and
it seems to work fine.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 3618d29c79a2..09292193dafe 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -258,6 +258,17 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
         return ret == 2 ? 0 : -1;
 }
 
+static bool drm_edid_is_zero(u8 *in_edid, int length)
+{
+        int i;
+        u32 *raw_edid = (u32 *)in_edid;
+
+        for (i = 0; i < length / 4; i++)
+                if (*(raw_edid + i) != 0)
+                        return false;
+        return true;
+}
+
 static u8 *
 drm_do_get_edid(struct drm_connector *connector, struct i2c_adapter *adapter)
 {
@@ -273,6 +284,10 @@ drm_do_get_edid(struct drm_connector *connector, struct i2c_adapter *adapter)
                         goto out;
                 if (drm_edid_block_valid(block))
                         break;
+                if (i == 0 && drm_edid_is_zero(block, EDID_LENGTH)) {
+                        connector->null_edid_counter++;
+                        goto carp;
+                }
         }
         if (i == 4)
                 goto carp;