Re: [PATCH v5 1/3] drm/msm/dp: cleared DP_DOWNSPREAD_CTRL register before start link training

[Kuogee Hsieh <quic_khsieh@quicinc.com>]

On 9/12/2022 11:39 AM, Dmitry Baryshkov wrote:
> On 12/09/2022 19:23, Kuogee Hsieh wrote:
>> DOWNSPREAD_CTRL (0x107) shall be cleared to 0 upon power-on reset or an
>> upstream device disconnect. This patch will enforce this rule by always
>> cleared DOWNSPREAD_CTRL register to 0 before start link training. At 
>> rare
>> case that DP MSA timing parameters may be mis-interpreted by the sink
>> which causes audio sampling rate be calculated wrongly and cause audio
>> did not work at sink if DOWNSPREAD_CTRL register is not cleared to 0.
>>
>> Changes in v2:
>> 1) fix spelling at commit text
>> 2) merge ssc variable into encoding[0]
>>
>> Changes in v3:
>> -- correct spelling of DOWNSPREAD_CTRL
>> -- replace err with len of ssize_t
>>
>> Changes in v4:
>> -- split into 2 patches
>>
>> Fixes: 154b5a7da0fd ("drm/msm/dp: add displayPort driver support")
>> Signed-off-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
>> ---
>>   drivers/gpu/drm/msm/dp/dp_ctrl.c | 13 +++++--------
>>   1 file changed, 5 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c 
>> b/drivers/gpu/drm/msm/dp/dp_ctrl.c
>> index ab6aa13..2c74c59 100644
>> --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
>> +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
>> @@ -1245,8 +1245,7 @@ static int dp_ctrl_link_train(struct 
>> dp_ctrl_private *ctrl,
>>   {
>>       int ret = 0;
>>       const u8 *dpcd = ctrl->panel->dpcd;
>> -    u8 encoding = DP_SET_ANSI_8B10B;
>> -    u8 ssc;
>> +    u8 encoding[] = { 0, DP_SET_ANSI_8B10B };
>>       u8 assr;
>>       struct dp_link_info link_info = {0};
>>   @@ -1258,13 +1257,11 @@ static int dp_ctrl_link_train(struct 
>> dp_ctrl_private *ctrl,
>>         dp_aux_link_configure(ctrl->aux, &link_info);
>>   -    if (drm_dp_max_downspread(dpcd)) {
>> -        ssc = DP_SPREAD_AMP_0_5;
>> -        drm_dp_dpcd_write(ctrl->aux, DP_DOWNSPREAD_CTRL, &ssc, 1);
>> -    }
>> +    if (drm_dp_max_downspread(dpcd))
>> +        encoding[0] |= DP_SPREAD_AMP_0_5;
>
> It would be simpler to call drm_dp_dpcd_write(ssc, DP_DOWNSPREAD_CTRL, 
> 1) unconditionally here. You won't have to change the 
> encoding/DP_MAIN_LINK_CHANNEL_CODING_SET/etc.

The difference is one write with 2 bytes against two writes with one 
byte each.

I think it is more efficient to combine two bytes into one write since 
these two bytes are consecutive address.

>
>>   -    drm_dp_dpcd_write(ctrl->aux, DP_MAIN_LINK_CHANNEL_CODING_SET,
>> -                &encoding, 1);
>> +    /* config DOWNSPREAD_CTRL and MAIN_LINK_CHANNEL_CODING_SET */
>> +    drm_dp_dpcd_write(ctrl->aux, DP_DOWNSPREAD_CTRL, encoding, 2);
>>         if (drm_dp_alternate_scrambler_reset_cap(dpcd)) {
>>           assr = DP_ALTERNATE_SCRAMBLER_RESET_ENABLE;
>