blob: 555c6f7f2e8c1287ff276419b107dcd10786441d [file] [log] [blame]
From c4e4fccc5d52d881afaac11d3353265ef4eccb8b Mon Sep 17 00:00:00 2001
From: Wayne Lin <>
Date: Fri, 3 Jan 2020 13:50:01 +0800
Subject: drm/dp_mst: correct the shifting in DP_REMOTE_I2C_READ
From: Wayne Lin <>
commit c4e4fccc5d52d881afaac11d3353265ef4eccb8b upstream.
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.
Correct the shifting value of NO_STOP_BIT for DP_REMOTE_I2C_READ case in
Changes since v1:(
* Add more descriptions in commit and cc to stable
Fixes: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")
Reviewed-by: Harry Wentland <>
Signed-off-by: Wayne Lin <>
Signed-off-by: Lyude Paul <>
Signed-off-by: Greg Kroah-Hartman <>
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
@@ -272,7 +272,7 @@ 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);