[PATCH v2.1 3/3] drm/edid: Switch DDC when reading the EDID

Lukas Wunner lukas at wunner.de
Fri Aug 14 09:28:35 PDT 2015

Originally by Seth Forshee <seth.forshee at canonical.com>, 2012-10-04:
Some dual graphics machines support muxing the DDC separately from the
display, so make use of this functionality when reading the EDID on the
inactive GPU. Also serialize drm_get_edid() with a mutex to avoid races
on the DDC mux state.

Modified by Dave Airlie <airlied at gmail.com>, 2012-12-22:
I can't figure out why I didn't like this, but I rewrote this [...] to
lock/unlock the ddc lines [...]. I think I'd prefer something like that
otherwise the interface got really ugly.

Modified by Lukas Wunner <lukas at wunner.de>, 2015-03-27:
Unlock DDC lines if drm_probe_ddc() fails.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88861
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61115
Tested-by: Pierre Moreau <pierre.morrow at free.fr>
    [MBP  5,3 2009  nvidia 9400M + 9600M GT   pre-retina]
Tested-by: Paul Hordiienko <pvt.gord at gmail.com>
    [MBP  6,2 2010  intel ILK + nvidia GT216  pre-retina]
Tested-by: William Brown <william at blackhats.net.au>
    [MBP  8,2 2011  intel SNB + amd turks     pre-retina]
Tested-by: Lukas Wunner <lukas at wunner.de>
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina]
Tested-by: Bruno Bierbaumer <bruno at bierbaumer.net>
    [MBP 11,3 2013  intel HSW + nvidia GK107  retina -- work in progress]

Cc: Seth Forshee <seth.forshee at canonical.com>
Cc: Dave Airlie <airlied at gmail.com>
Signed-off-by: Lukas Wunner <lukas at wunner.de>
 drivers/gpu/drm/drm_edid.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index e6e05bb..cdb2fa1 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -32,6 +32,7 @@
 #include <linux/hdmi.h>
 #include <linux/i2c.h>
 #include <linux/module.h>
+#include <linux/vga_switcheroo.h>
 #include <drm/drmP.h>
 #include <drm/drm_edid.h>
 #include <drm/drm_displayid.h>
@@ -1377,13 +1378,21 @@ struct edid *drm_get_edid(struct drm_connector *connector,
 			  struct i2c_adapter *adapter)
 	struct edid *edid;
+	struct pci_dev *pdev = connector->dev->pdev;
-	if (!drm_probe_ddc(adapter))
+	vga_switcheroo_lock_ddc(pdev);
+	if (!drm_probe_ddc(adapter)) {
+		vga_switcheroo_unlock_ddc(pdev);
 		return NULL;
+	}
 	edid = drm_do_get_edid(connector, drm_do_probe_ddc_edid, adapter);
 	if (edid)
 		drm_get_displayid(connector, edid);
+	vga_switcheroo_unlock_ddc(pdev);
 	return edid;

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