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

[Harry Wentland]

On 2020-01-03 12:50 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.
> 
> 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.
> 

Thanks for elaborating on the potential consequences of this bug.

Harry

> [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++;
>  		}
>