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@amd.com Signed-off-by: Wayne Lin Wayne.Lin@amd.com Cc: stable@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++;
  

  }

Oh! Just a friendly tip, I fixed this before pushing your patch:

➜ drm-maint git:(drm-misc-fixes) dim push dim: 0b1d54cedbb4 ("drm/dp_mst: correct the shifting in DP_REMOTE_I2C_READ"): Fixes: SHA1 needs at least 12 digits: dim: ad7f8a1f9ce (drm/helper: add Displayport multi-stream helper (v0.6)) dim: ERROR: issues in commits detected, aborting

For the future, we have a set of DRM maintainer tools (they're quite useful for people who aren't maintainers though!) that you can use to make sure your patch is formatted correctly ahead of time:

https://drm.pages.freedesktop.org/maintainer-tools/dim.html

Particularly useful commands: * dim sparse # Checks for trivial code issues, like set but unused variables * dim checkpatch # Checks for code style issues * dim fixes $COMMIT_ID # Adds an appropriately formatted Fixes: tag * dim cite $COMMIT_ID # Adds an appropriately formatted Reference: tag

On Fri, 2020-01-03 at 13:50 +0800, 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.

[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@amd.com Signed-off-by: Wayne Lin Wayne.Lin@amd.com Cc: stable@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++;
}