[PATCH] drm/bridge: sii902x: fix get edid may fail

[Andrea Merello]

From: Andrea Merello <andrea.merello at gmail.com>

The standard DRM function to get the edid from the i2c bus performs
(at least) two transfers.

By experiments it seems that the sii9022a have problems with the
2nd I2C start, at least unless a wait is introduced detween the
two transfers.

So we perform one single I2C transfer, and if the transfer must be
split, then we introduce a delay.

Signed-off-by: Andrea Merello <andrea.merello at iit.it>
Cc: Andrea Merello <andrea.merello at gmail.com>
Cc: Boris Brezillon <boris.brezillon at free-electrons.com>
Cc: Archit Taneja <architt at codeaurora.org>
Cc: David Airlie <airlied at linux.ie>
---
 drivers/gpu/drm/bridge/sii902x.c | 70 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 69 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/sii902x.c b/drivers/gpu/drm/bridge/sii902x.c
index 9126d03..042d7e2 100644
--- a/drivers/gpu/drm/bridge/sii902x.c
+++ b/drivers/gpu/drm/bridge/sii902x.c
@@ -133,6 +133,62 @@ static const struct drm_connector_funcs sii902x_connector_funcs = {
 	.atomic_destroy_state = drm_atomic_helper_connector_destroy_state,
 };
 
+#define DDC_SEGMENT_ADDR 0x30
+static int sii902x_do_probe_ddc_edid(void *data, u8 *buf,
+				     unsigned int block, size_t len)
+{
+	struct i2c_adapter *adapter = data;
+	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
+	 * adapter reports EAGAIN. However, we find that bit-banging transfers
+	 * are susceptible to errors under a heavily loaded machine and
+	 * generate spurious NAKs and timeouts. Retrying the transfer
+	 * of the individual block a few times seems to overcome this.
+	 */
+	while (1) {
+		struct i2c_msg msgs[] = {
+			{
+				.addr	= DDC_SEGMENT_ADDR,
+				.flags	= 0,
+				.len	= 1,
+				.buf	= &segment,
+			}, {
+				.addr	= DDC_ADDR,
+				.flags	= 0,
+				.len	= 1,
+				.buf	= &start,
+			}, {
+				.addr	= DDC_ADDR,
+				.flags	= I2C_M_RD,
+				.len	= len,
+				.buf	= buf,
+			}
+		};
+
+		/*
+		 * Avoid sending the segment addr to not upset non-compliant
+		 * DDC monitors.
+		 */
+		ret = i2c_transfer(adapter, &msgs[3 - xfers], xfers);
+
+		if (ret == -ENXIO) {
+			DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
+				      adapter->name);
+			break;
+		}
+		if (ret == xfers || --retries == 0)
+			break;
+
+		udelay(100);
+	}
+
+	return ret == xfers ? 0 : -1;
+}
 static int sii902x_get_modes(struct drm_connector *connector)
 {
 	struct sii902x *sii902x = connector_to_sii902x(connector);
@@ -168,8 +224,20 @@ static int sii902x_get_modes(struct drm_connector *connector)
 	if (ret)
 		return ret;
 
-	edid = drm_get_edid(connector, sii902x->i2c->adapter);
+	/* drm_get_edid() runs two I2C transfers. The sii902x seems
+	 * to have problem with the 2nd I2C start. A wait seems needed.
+	 * So, we don't perform use drm_get_edid(). We don't perform
+	 * the first "probe" transfer, and we use a custom block read
+	 * function that, in case the trasfer is split, does introduce
+	 * a delay.
+	 */
+	edid = drm_do_get_edid(connector, sii902x_do_probe_ddc_edid,
+			       sii902x->i2c->adapter);
+	if (!edid)
+		return num;
+
 	drm_mode_connector_update_edid_property(connector, edid);
+
 	if (edid) {
 		num = drm_add_edid_modes(connector, edid);
 		kfree(edid);
-- 
2.7.4