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

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Date:   Mon, 12 Sep 2022 22:25:00 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     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_abhinavk@...cinc.com, 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 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")
>>> 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;
>>

-- 
With best wishes
Dmitry