[PATCH V3] 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 |   83 ++++++++++++++++++++++++++++++++-----------
 1 files changed, 62 insertions(+), 21 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index a8743c3..a10a130 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 short segFlags = segment ? 0 : I2C_M_IGNORE_NAK;
 	int ret, retries = 5;
 
 	/* The core i2c driver will automatically retry the transfer if the
@@ -262,29 +264,68 @@ drm_do_probe_ddc_edid(struct i2c_adapter *adapter, unsigned char *buf,
 	 * generate spurious NAKs and timeouts. Retrying the transfer
 	 * of the individual block a few times seems to overcome this.
 	 */
-	do {
-		struct i2c_msg msgs[] = {
-			{
-				.addr	= DDC_ADDR,
-				.flags	= 0,
-				.len	= 1,
-				.buf	= &start,
-			}, {
-				.addr	= DDC_ADDR,
-				.flags	= I2C_M_RD,
-				.len	= len,
-				.buf	= buf,
+	switch (segment) {
+		/* To reduce traffic on I2C, we request the i2c xfer
+		 * for segment addr only if 4 block edid data is
+		 * present.
+		 * 0 : DDC
+		 * 1 : E-DDC
+		 */
+	case 0:
+		do {
+			struct i2c_msg msgs[] = {
+				{
+					.addr	= DDC_ADDR,
+					.flags	= 0,
+					.len	= 1,
+					.buf	= &start,
+				}, {
+					.addr	= DDC_ADDR,
+					.flags	= I2C_M_RD,
+					.len	= len,
+					.buf	= buf,
+				}
+			};
+			ret = i2c_transfer(adapter, msgs, 2);
+			if (ret == -ENXIO) {
+				DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
+							adapter->name);
+				break;
 			}
-		};
-		ret = i2c_transfer(adapter, msgs, 2);
-		if (ret == -ENXIO) {
-			DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
-					adapter->name);
-			break;
-		}
-	} while (ret != 2 && --retries);
+		} while (ret != 2 && --retries);
+
+		return ret == 2 ? 0 : -1;
+	case 1:
+		do {
+			struct i2c_msg msgs[] = {
+				{ /*set segment pointer */
+					.addr	= DDC_SEGMENT_ADDR,
+					.flags	= segFlags,
+					.len	= 1,
+					.buf	= &segment,
+				}, { /*set offset */
+					.addr	= DDC_ADDR,
+					.flags	= 0,
+					.len	= 1,
+					.buf	= &start,
+				}, { /*set data */
+					.addr	= DDC_ADDR,
+					.flags	= I2C_M_RD,
+					.len	= len,
+					.buf	= buf,
+				}
+			};
+			ret = i2c_transfer(adapter, msgs, 3);
+			if (ret == -ENXIO) {
+				DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
+							adapter->name);
+				break;
+			}
+		} while (ret != 3 && --retries);
 
-	return ret == 2 ? 0 : -1;
+		return ret == 3 ? 0 : -1;
+	}
+	return -1;
 }
 
 static bool drm_edid_is_zero(u8 *in_edid, int length)
-- 
1.7.0.4