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

[Harry Wentland]

On 2019-12-30 2:05 a.m., Wayne Lin wrote:
> [Why]
> According to DP spec, it should shift left 4 digits for NO_STOP_BIT
> in REMOTE_I2C_READ message. Not 5 digits.
> 
> [How]
> Correct the shifting value of NO_STOP_BIT for DP_REMOTE_I2C_READ case in
> drm_dp_encode_sideband_req().
> 
> Signed-off-by: Wayne Lin <Wayne.Lin at amd.com>

Good catch. Looks like this has been there since the beginning of MST in
the kernel. How did you find this? Did this cause bad EDID reads or some
other problem? If so the commit message should probably mention it.

Harry

> ---
>  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 1d1bfa49ca2b..0557e225ff67 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++;
>  		}
>