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

[Harry Wentland]

On 2019-12-30 11:00 p.m., Lin, Wayne wrote:
> [AMD Official Use Only - Internal Distribution Only]
> 

Make sure to select the correct AIP designation for public emails. See
my email on that from yesterday. :)

>> ________________________________________
>> From: Wentland, Harry <Harry.Wentland at amd.com>
>> Sent: Monday, December 30, 2019 23:26
>> To: Lin, Wayne; dri-devel at lists.freedesktop.org; amd-gfx at lists.freedesktop.org
>> Cc: lyude at 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:
>>> [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 at 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. Was just curious if you saw any real issues. Good find
nevertheless and good patch.

You can add my
Reviewed-by: Harry Wentland <harry.wentland at amd.com>

Harry

> Thanks.
>>
>>> ---
>>>  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
>