From: Wayne Lin <Wayne.Lin@xxxxxxx> commit c4e4fccc5d52d881afaac11d3353265ef4eccb8b upstream. [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: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)") Reviewed-by: Harry Wentland <harry.wentland@xxxxxxx> Signed-off-by: Wayne Lin <Wayne.Lin@xxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx> Link: https://patchwork.freedesktop.org/patch/msgid/20200103055001.10287-1-Wayne.Lin@xxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/gpu/drm/drm_dp_mst_topology.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/drivers/gpu/drm/drm_dp_mst_topology.c +++ b/drivers/gpu/drm/drm_dp_mst_topology.c @@ -339,7 +339,7 @@ static void drm_dp_encode_sideband_req(s 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++; }
