linux-kernel - Re: [PATCH v5 1/3] drm/msm/dp: cleared DP_DOWNSPREAD

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Date:   Thu, 29 Sep 2022 18:36:53 -0700
From:   Abhinav Kumar <quic_abhinavk@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>,
        <dri-devel@...ts.freedesktop.org>, <robdclark@...il.com>,
        <sean@...rly.run>, <swboyd@...omium.org>, <dianders@...omium.org>,
        <vkoul@...nel.org>, <daniel@...ll.ch>, <airlied@...ux.ie>,
        <agross@...nel.org>, <bjorn.andersson@...aro.org>
CC:     <quic_sbillaka@...cinc.com>, <freedreno@...ts.freedesktop.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/3] drm/msm/dp: cleared DP_DOWNSPREAD_CTRL register
 before start link training



On 9/12/2022 12:25 PM, Dmitry Baryshkov wrote:
> On 12/09/2022 22:21, Kuogee Hsieh wrote:
>>
>> 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")

Fixes tag is wrong here. It should be:

Fixes: c943b4948b58 ("drm/msm/dp: add displayPort driver support")

>>>> Signed-off-by: Kuogee Hsieh <quic_khsieh@...cinc.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.
> 
> I probably wouldn't do so, nevertheless:
> 
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> 
>>
>>>
>>>>   -    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;
>>>
>