drm/dp: Use large transactions for I2C over AUX

Older DisplayPort to DVI-D Dual Link adapters designed by Bizlink have bugs
in their I2C over AUX implementation (fixed in newer revisions). 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. For this purpose, use the algorithm in the DisplayPort 1.2 spec,
in the hopes that it'll be closest to what Windows does.

Also provide an unsafe module parameter for testing smaller transfer sizes,
in case there are sinks out there that cannot work with Windows.

Note also that despite the previous comment in drm_dp_i2c_xfer, this speeds
up native DP EDID reads; Ville Syrjälä <ville.syrjala@linux.intel.com> found
the following changes in his testing:

Device under test:     old  -> with this patch
DP->DVI (OUI 001cf8):  40ms -> 35ms
DP->VGA (OUI 0022b9):  45ms -> 38ms
Zotac DP->2xHDMI:      25ms ->  4ms
Asus PB278 monitor:    22ms ->  3ms

A back of the envelope calculation shows that peak theoretical transfer rate
for 1 byte reads is around 60 kbit/s; with 16 byte reads, this increases to
around 500 kbit/s, which explains the increase in speed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55228
Tested-by: Aidan Marks <aidanamarks@gmail.com> (v3)
Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_dp_helper.c

@@ -427,11 +427,13 @@ 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
@@ -440,14 +442,14 @@ 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;
 		}
 
 
@@ -488,9 +490,7 @@ 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");
@@ -513,14 +513,55 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 	return -EREMOTEIO;
 }
 
+/*
+ * Keep retrying drm_dp_i2c_do_msg until all data has been transferred.
+ *
+ * Returns an error code on failure, or a recommended transfer size on success.
+ */
+static int drm_dp_i2c_drain_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *orig_msg)
+{
+	int err, ret = orig_msg->size;
+	struct drm_dp_aux_msg msg = *orig_msg;
+
+	while (msg.size > 0) {
+		err = drm_dp_i2c_do_msg(aux, &msg);
+		if (err <= 0)
+			return err == 0 ? -EPROTO : err;
+
+		if (err < msg.size && err < ret) {
+			DRM_DEBUG_KMS("Partial I2C reply: requested %zu bytes got %d bytes\n",
+				      msg.size, err);
+			ret = err;
+		}
+
+		msg.size -= err;
+		msg.buffer += err;
+	}
+
+	return ret;
+}
+
+/*
+ * 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. Hence the default is maximum.
+ */
+static int dp_aux_i2c_transfer_size __read_mostly = DP_AUX_MAX_PAYLOAD_BYTES;
+module_param_unsafe(dp_aux_i2c_transfer_size, int, 0644);
+MODULE_PARM_DESC(dp_aux_i2c_transfer_size,
+		 "Number of bytes to transfer in a single I2C over DP AUX CH message, (1-16, default 16)");
+
 static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 			   int num)
 {
 	struct drm_dp_aux *aux = adapter->algo_data;
 	unsigned int i, j;
+	unsigned transfer_size;
 	struct drm_dp_aux_msg msg;
 	int err = 0;
 
+	dp_aux_i2c_transfer_size = clamp(dp_aux_i2c_transfer_size, 1, DP_AUX_MAX_PAYLOAD_BYTES);
+
 	memset(&msg, 0, sizeof(msg));
 
 	for (i = 0; i < num; i++) {
@@ -538,20 +579,19 @@ 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.
+		/* We want each transaction to be as large as possible, but
+		 * we'll go to smaller sizes if the hardware gives us a
+		 * short reply.
 		 */
-		for (j = 0; j < msgs[i].len; j++) {
+		transfer_size = dp_aux_i2c_transfer_size;
+		for (j = 0; j < msgs[i].len; j += msg.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);
+			err = drm_dp_i2c_drain_msg(aux, &msg);
 			if (err < 0)
 				break;
+			transfer_size = err;
 		}
 		if (err < 0)
 			break;

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
@@ -680,6 +682,9 @@ 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