[PATCH] drm/i915: Fix single msg gmbus_xfers writes

[Benson Leung <bleung@chromium.org>]

gmbus_xfer with a single message (particularly a single message write) would
set Bus Cycle Select to 100b, the Gen Stop cycle, instead of 101b,
No Index, Stop cycle. This would not start single message i2c transactions.

Also, gmbus_xfer done: will disable the interface without checking if
it is idle. In the case of writes, there will be no wait on status or delay
to ensure the write starts and completes before the interface is turned off.

Fixed the former issue by using the same cycle selection as used in the
I2C_M_RD for the write case.
GMBUS_CYCLE_WAIT | (i + 1 == num ? GMBUS_CYCLE_STOP : 0)
Fixed the latter by waiting on GMBUS_ACTIVE to deassert before disable.

Signed-off-by: Benson Leung <bleung@chromium.org>
---
 drivers/gpu/drm/i915/intel_i2c.c |   13 +++++++++----
 1 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index d30cccc..64bb9cd 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -249,7 +249,8 @@ gmbus_xfer(struct i2c_adapter *adapter,
 
 		if (msgs[i].flags & I2C_M_RD) {
 			I915_WRITE(GMBUS1 + reg_offset,
-				   GMBUS_CYCLE_WAIT | (i + 1 == num ? GMBUS_CYCLE_STOP : 0) |
+				   GMBUS_CYCLE_WAIT |
+				   (i + 1 == num ? GMBUS_CYCLE_STOP : 0) |
 				   (len << GMBUS_BYTE_COUNT_SHIFT) |
 				   (msgs[i].addr << GMBUS_SLAVE_ADDR_SHIFT) |
 				   GMBUS_SLAVE_READ | GMBUS_SW_RDY);
@@ -278,7 +279,8 @@ gmbus_xfer(struct i2c_adapter *adapter,
 
 			I915_WRITE(GMBUS3 + reg_offset, val);
 			I915_WRITE(GMBUS1 + reg_offset,
-				   (i + 1 == num ? GMBUS_CYCLE_STOP : GMBUS_CYCLE_WAIT) |
+				   GMBUS_CYCLE_WAIT |
+				   (i + 1 == num ? GMBUS_CYCLE_STOP : 0) |
 				   (msgs[i].len << GMBUS_BYTE_COUNT_SHIFT) |
 				   (msgs[i].addr << GMBUS_SLAVE_ADDR_SHIFT) |
 				   GMBUS_SLAVE_WRITE | GMBUS_SW_RDY);
@@ -317,9 +319,12 @@ clear_err:
 	I915_WRITE(GMBUS1 + reg_offset, 0);
 
 done:
-	/* Mark the GMBUS interface as disabled. We will re-enable it at the
-	 * start of the next xfer, till then let it sleep.
+	/* Mark the GMBUS interface as disabled after waiting for idle.
+	 * We will re-enable it at the start of the next xfer,
+	 * till then let it sleep.
 	 */
+	if (wait_for((I915_READ(GMBUS2 + reg_offset) & GMBUS_ACTIVE) == 0, 10))
+		DRM_INFO("GMBUS timed out waiting for idle\n");
 	I915_WRITE(GMBUS0 + reg_offset, 0);
 	return i;
 
-- 
1.7.1