[PATCH V4] drm: edid: add support for E-DDC

[Shirish S]

The current logic for probing ddc is limited to
2 blocks (256 bytes), this patch adds support
for the 4 block (512) data.

To do this, a single 8-bit segment index is
passed to the display via the I2C address 30h.
Data from the selected segment is then immediately
read via the regular DDC2 address using a repeated
I2C 'START' signal.

Signed-off-by: Shirish S <s.shirish at samsung.com>
---
 drivers/gpu/drm/drm_edid.c |   22 ++++++++++++++++++----
 1 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index a8743c3..cde7af0 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -254,6 +254,8 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
 		      int block, int len)
 {
 	unsigned char start = block * EDID_LENGTH;
+	unsigned char segment = block >> 1;
+	unsigned char xfers = segment ? 3 : 2;
 	int ret, retries = 5;
 
 	/* The core i2c driver will automatically retry the transfer if the
@@ -264,7 +266,12 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
 	 */
 	do {
 		struct i2c_msg msgs[] = {
-			{
+			{ /*set segment pointer */
+				.addr	= DDC_SEGMENT_ADDR,
+				.flags	= segment ? 0 : I2C_M_IGNORE_NAK,
+				.len	= 1,
+				.buf	= &segment,
+			}, {
 				.addr	= DDC_ADDR,
 				.flags	= 0,
 				.len	= 1,
@@ -276,15 +283,22 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
 				.buf	= buf,
 			}
 		};
-		ret = i2c_transfer(adapter, msgs, 2);
+	/* Avoid sending the segment addr to not upset non-compliant ddc
+	 * monitors.
+	 */
+		if (!segment)
+			ret = i2c_transfer(adapter, &msgs[1], xfers);
+		else
+			ret = i2c_transfer(adapter, msgs, xfers);
+
 		if (ret == -ENXIO) {
 			DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
 					adapter->name);
 			break;
 		}
-	} while (ret != 2 && --retries);
+	} while (ret != xfers && --retries);
 
-	return ret == 2 ? 0 : -1;
+	return ret == xfers ? 0 : -1;
 }
 
 static bool drm_edid_is_zero(u8 *in_edid, int length)
-- 
1.7.0.4