drm/i915: Make use of indexed write GMBUS feature

This patch enables the indexed write feature of the GMBUS to concatenate
2 consecutive messages into one. The criteria for an indexed write is
that both messages are writes, the first is length == 1, and the second
is length > 0. The first message is sent out by the GMBUS as the slave
command, and the second one is sent via the GMBUS FIFO as usual.

Changes in v3:
- Added to series
Changes in v4:
- Combine indexed reads and writes (Ville)
Changes in v5:
- checkpatch whitespace nits
Changes in v6:
- None
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180108195545.218615-7-seanpaul@chromium.org

drivers/gpu/drm/i915/intel_i2c.c

@@ -402,7 +402,8 @@ gmbus_xfer_read(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
 
 static int
 gmbus_xfer_write_chunk(struct drm_i915_private *dev_priv,
-		       unsigned short addr, u8 *buf, unsigned int len)
+		       unsigned short addr, u8 *buf, unsigned int len,
+		       u32 gmbus1_index)
 {
 	unsigned int chunk_size = len;
 	u32 val, loop;
@@ -415,7 +416,7 @@ gmbus_xfer_write_chunk(struct drm_i915_private *dev_priv,
 
 	I915_WRITE_FW(GMBUS3, val);
 	I915_WRITE_FW(GMBUS1,
-		      GMBUS_CYCLE_WAIT |
+		      gmbus1_index | GMBUS_CYCLE_WAIT |
 		      (chunk_size << GMBUS_BYTE_COUNT_SHIFT) |
 		      (addr << GMBUS_SLAVE_ADDR_SHIFT) |
 		      GMBUS_SLAVE_WRITE | GMBUS_SW_RDY);
@@ -438,7 +439,8 @@ gmbus_xfer_write_chunk(struct drm_i915_private *dev_priv,
 }
 
 static int
-gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg)
+gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg,
+		 u32 gmbus1_index)
 {
 	u8 *buf = msg->buf;
 	unsigned int tx_size = msg->len;
@@ -448,7 +450,8 @@ gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg)
 	do {
 		len = min(tx_size, GMBUS_BYTE_COUNT_MAX);
 
-		ret = gmbus_xfer_write_chunk(dev_priv, msg->addr, buf, len);
+		ret = gmbus_xfer_write_chunk(dev_priv, msg->addr, buf, len,
+					     gmbus1_index);
 		if (ret)
 			return ret;
 
@@ -460,21 +463,21 @@ gmbus_xfer_write(struct drm_i915_private *dev_priv, struct i2c_msg *msg)
 }
 
 /*
- * The gmbus controller can combine a 1 or 2 byte write with a read that
- * immediately follows it by using an "INDEX" cycle.
+ * The gmbus controller can combine a 1 or 2 byte write with another read/write
+ * that immediately follows it by using an "INDEX" cycle.
  */
 static bool
-gmbus_is_index_read(struct i2c_msg *msgs, int i, int num)
+gmbus_is_index_xfer(struct i2c_msg *msgs, int i, int num)
 {
 	return (i + 1 < num &&
 		msgs[i].addr == msgs[i + 1].addr &&
 		!(msgs[i].flags & I2C_M_RD) &&
 		(msgs[i].len == 1 || msgs[i].len == 2) &&
-		(msgs[i + 1].flags & I2C_M_RD));
+		msgs[i + 1].len > 0);
 }
 
 static int
-gmbus_xfer_index_read(struct drm_i915_private *dev_priv, struct i2c_msg *msgs)
+gmbus_index_xfer(struct drm_i915_private *dev_priv, struct i2c_msg *msgs)
 {
 	u32 gmbus1_index = 0;
 	u32 gmbus5 = 0;
@@ -491,7 +494,10 @@ gmbus_xfer_index_read(struct drm_i915_private *dev_priv, struct i2c_msg *msgs)
 	if (gmbus5)
 		I915_WRITE_FW(GMBUS5, gmbus5);
 
+	if (msgs[1].flags & I2C_M_RD)
 		ret = gmbus_xfer_read(dev_priv, &msgs[1], gmbus1_index);
+	else
+		ret = gmbus_xfer_write(dev_priv, &msgs[1], gmbus1_index);
 
 	/* Clear GMBUS5 after each index transfer */
 	if (gmbus5)
@@ -522,13 +528,13 @@ do_gmbus_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs, int num)
 
 	for (; i < num; i += inc) {
 		inc = 1;
-		if (gmbus_is_index_read(msgs, i, num)) {
-			ret = gmbus_xfer_index_read(dev_priv, &msgs[i]);
-			inc = 2; /* an index read is two msgs */
+		if (gmbus_is_index_xfer(msgs, i, num)) {
+			ret = gmbus_index_xfer(dev_priv, &msgs[i]);
+			inc = 2; /* an index transmission is two msgs */
 		} else if (msgs[i].flags & I2C_M_RD) {
 			ret = gmbus_xfer_read(dev_priv, &msgs[i], 0);
 		} else {
-			ret = gmbus_xfer_write(dev_priv, &msgs[i]);
+			ret = gmbus_xfer_write(dev_priv, &msgs[i], 0);
 		}
 
 		if (!ret)