[PATCH] drm/dp: Use large transactions for I2C over AUX

[Simon Farnsworth <simon.farnsworth@onelan.co.uk>]

DisplayPort to DVI-D Dual Link adapters designed by Bizlink have bugs in
their I2C over AUX implementation. They work fine with Windows, but fail
with Linux.

It turns out that they cannot keep an I2C transaction open unless the
previous read was 16 bytes; shorter reads can only be followed by a zero
byte transfer ending the I2C transaction.

Copy Windows's behaviour, and read 16 bytes at a time. If we get a short
reply, assume that there's a hardware bottleneck, and shrink our read size
to match.

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
---

v2 changes, after feedback from Thierry and Ville:

 * Handle short replies. I've decided (arbitrarily) that a short reply
   results in us dropping back to the newly chosen size for the rest of this
   I2C transaction. Thus, given an attempt to read the first 16 bytes of
   EDID, and a sink that only does 4 bytes of buffering, we will see the
   following AUX transfers for the EDID read (after address is set):

   <set address, block etc>
   Read 16 bytes from I2C over AUX.
   Reply with 4 bytes
   Read 4 bytes
   Reply with 4 bytes
   Read 4 bytes
   Reply with 4 bytes
   Read 4 bytes
   Reply with 4 bytes
   <end I2C transaction>

Note that I've not looked at MST support - I have neither the DP 1.2 spec
nor any MST branch devices, so I can't test anything I write or check it
against a spec. It looks from the code, however, as if MST has the branch
device do the split from a big request into small transactions.

 drivers/gpu/drm/drm_dp_helper.c | 42 ++++++++++++++++++++++-------------------
 include/drm/drm_dp_helper.h     |  5 +++++
 2 files changed, 28 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 79968e3..701b201 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -396,11 +396,13 @@ static u32 drm_dp_i2c_functionality(struct i2c_adapter *adapter)
  * retrying the transaction as appropriate.  It is assumed that the
  * aux->transfer function does not modify anything in the msg other than the
  * reply field.
+ *
+ * Returns bytes transferred on success, or a negative error code on failure.
  */
 static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 {
 	unsigned int retry;
-	int err;
+	int ret;
 
 	/*
 	 * DP1.2 sections 2.7.7.1.5.6.1 and 2.7.7.1.6.6.1: A DP Source device
@@ -409,14 +411,14 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 	 */
 	for (retry = 0; retry < 7; retry++) {
 		mutex_lock(&aux->hw_mutex);
-		err = aux->transfer(aux, msg);
+		ret = aux->transfer(aux, msg);
 		mutex_unlock(&aux->hw_mutex);
-		if (err < 0) {
-			if (err == -EBUSY)
+		if (ret < 0) {
+			if (ret == -EBUSY)
 				continue;
 
-			DRM_DEBUG_KMS("transaction failed: %d\n", err);
-			return err;
+			DRM_DEBUG_KMS("transaction failed: %d\n", ret);
+			return ret;
 		}
 
 
@@ -457,9 +459,7 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 			 * Both native ACK and I2C ACK replies received. We
 			 * can assume the transfer was successful.
 			 */
-			if (err < msg->size)
-				return -EPROTO;
-			return 0;
+			return ret;
 
 		case DP_AUX_I2C_REPLY_NACK:
 			DRM_DEBUG_KMS("I2C nack\n");
@@ -487,6 +487,7 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 {
 	struct drm_dp_aux *aux = adapter->algo_data;
 	unsigned int i, j;
+	int transfer_size = DP_AUX_MAX_PAYLOAD_BYTES;
 	struct drm_dp_aux_msg msg;
 	int err = 0;
 
@@ -507,20 +508,23 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 		err = drm_dp_i2c_do_msg(aux, &msg);
 		if (err < 0)
 			break;
-		/*
-		 * Many hardware implementations support FIFOs larger than a
-		 * single byte, but it has been empirically determined that
-		 * transferring data in larger chunks can actually lead to
-		 * decreased performance. Therefore each message is simply
-		 * transferred byte-by-byte.
+		/* Bizlink designed DP->DVI-D Dual Link adapters require the
+                 * I2C over AUX packets to be as large as possible. If not,
+                 * the I2C transactions never succeed.
+		 *
+		 * We therefore start by requesting 16 byte transfers. If
+		 * the hardware gives us a partial ACK, we stick to the new
+		 * smaller size from the partial ACK.
 		 */
-		for (j = 0; j < msgs[i].len; j++) {
+		for (j = 0; j < msgs[i].len; j += transfer_size) {
 			msg.buffer = msgs[i].buf + j;
-			msg.size = 1;
+			msg.size = min(transfer_size, msgs[i].len - j);
 
-			err = drm_dp_i2c_do_msg(aux, &msg);
-			if (err < 0)
+			transfer_size = drm_dp_i2c_do_msg(aux, &msg);
+			if (transfer_size <= 0) {
+				err = transfer_size == 0 ? -EPROTO : transfer_size;
 				break;
+			}
 		}
 		if (err < 0)
 			break;
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 11f8c84..444d51b 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -42,6 +42,8 @@
  * 1.2 formally includes both eDP and DPI definitions.
  */
 
+#define DP_AUX_MAX_PAYLOAD_BYTES	16
+
 #define DP_AUX_I2C_WRITE		0x0
 #define DP_AUX_I2C_READ			0x1
 #define DP_AUX_I2C_STATUS		0x2
@@ -519,6 +521,9 @@ struct drm_dp_aux_msg {
  * transactions. The drm_dp_aux_register_i2c_bus() function registers an
  * I2C adapter that can be passed to drm_probe_ddc(). Upon removal, drivers
  * should call drm_dp_aux_unregister_i2c_bus() to remove the I2C adapter.
+ * The I2C adapter uses long transfers by default; if a partial response is
+ * received, the adapter will drop down to the size given by the partial
+ * response for this transaction only.
  *
  * Note that the aux helper code assumes that the .transfer() function
  * only modifies the reply field of the drm_dp_aux_msg structure.  The
-- 
2.1.0

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