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

[AMD Official Use Only - Internal Distribution Only]

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From: Wentland, Harry Harry.Wentland@amd.com Sent: Monday, December 30, 2019 23:26 To: Lin, Wayne; dri-devel@lists.freedesktop.org; amd-gfx@lists.freedesktop.org Cc: lyude@redhat.com; Zuo, Jerry; Kazlauskas, Nicholas; Wentland, Harry Subject: Re: [PATCH] drm/dp_mst: correct the shifting in DP_REMOTE_I2C_READ

On 2019-12-30 2:05 a.m., Wayne Lin wrote:

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[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@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

Thanks for your time.

I found this while I was trying to debug MST issues by using AUX monitor. So far, I don't observe problems that relate to this under my test cases and environment. However, this bit might affect the I2C signal generated by I2C master, I will mention more in the commit message.

Thanks.

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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++;
          }
  
  

-- Wayne Lin