[PATCH v2] drm/dp_mst: correct the shifting in DP_REMOTE_I2C_READ

[Wayne Lin]

[Why]
According to DP spec, it should shift left 4 digits for NO_STOP_BIT
in REMOTE_I2C_READ message. Not 5 digits.

In current code, NO_STOP_BIT is always set to zero which means I2C
master is always generating a I2C stop at the end of each I2C write
transaction while handling REMOTE_I2C_READ sideband message. This issue
might have the generated I2C signal not meeting the requirement. Take
random read in I2C for instance, I2C master should generate a repeat
start to start to read data after writing the read address. This issue
will cause the I2C master to generate a stop-start rather than a
re-start which is not expected in I2C random read.

[How]
Correct the shifting value of NO_STOP_BIT for DP_REMOTE_I2C_READ case in
drm_dp_encode_sideband_req().

Changes since v1:(https://patchwork.kernel.org/patch/11312667/)
* Add more descriptions in commit and cc to stable

Fixes: ad7f8a1f9ce (drm/helper: add Displayport multi-stream helper (v0.6))
Reviewed-by: Harry Wentland <harry.wentland at amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin at amd.com>
Cc: stable at vger.kernel.org
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 1cf5f8b8bbb8..9d24c98bece1 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -393,7 +393,7 @@ drm_dp_encode_sideband_req(const struct drm_dp_sideband_msg_req_body *req,
 			memcpy(&buf[idx], req->u.i2c_read.transactions[i].bytes, req->u.i2c_read.transactions[i].num_bytes);
 			idx += req->u.i2c_read.transactions[i].num_bytes;
 
-			buf[idx] = (req->u.i2c_read.transactions[i].no_stop_bit & 0x1) << 5;
+			buf[idx] = (req->u.i2c_read.transactions[i].no_stop_bit & 0x1) << 4;
 			buf[idx] |= (req->u.i2c_read.transactions[i].i2c_transaction_delay & 0xf);
 			idx++;
 		}
-- 
2.17.1