linux-kernel - Re: [PATCH v1 2/3] drm/msm/dp: decoupling dp->id out of dp controller_id at scxxxx_dp

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Message-ID: <fa7f8bf1-33cd-5515-0143-6596df2bd740@quicinc.com>
Date:   Fri, 24 Jun 2022 14:49:57 -0700
From:   Kuogee Hsieh <quic_khsieh@...cinc.com>
To:     Stephen Boyd <swboyd@...omium.org>, <agross@...nel.org>,
        <airlied@...ux.ie>, <bjorn.andersson@...aro.org>,
        <daniel@...ll.ch>, <dianders@...omium.org>,
        <dmitry.baryshkov@...aro.org>, <dri-devel@...ts.freedesktop.org>,
        <robdclark@...il.com>, <sean@...rly.run>, <vkoul@...nel.org>
CC:     <quic_abhinavk@...cinc.com>, <quic_aravindh@...cinc.com>,
        <quic_sbillaka@...cinc.com>, <freedreno@...ts.freedesktop.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 2/3] drm/msm/dp: decoupling dp->id out of dp
 controller_id at scxxxx_dp_cfg table


On 6/24/2022 2:40 PM, Stephen Boyd wrote:
> Quoting Kuogee Hsieh (2022-06-24 14:17:50)
>> On 6/24/2022 1:00 PM, Stephen Boyd wrote:
>>> Quoting Kuogee Hsieh (2022-06-24 10:15:11)
>>>> Current the index (dp->id) of DP descriptor table (scxxxx_dp_cfg[]) are tightly
>>>> coupled with DP controller_id. This means DP use controller id 0 must be placed
>>>> at first entry of DP descriptor table (scxxxx_dp_cfg[]). Otherwise the internal
>>>> INTF will mismatch controller_id. This will cause controller kickoff wrong
>>>> interface timing engine and cause dpu_encoder_phys_vid_wait_for_commit_done
>>>> vblank timeout error.
>>>>
>>>> This patch add controller_id field into struct msm_dp_desc to break the tightly
>>>> coupled relationship between index (dp->id) of DP descriptor table with DP
>>>> controller_id.
>>> Please no. This reverts the intention of commit bb3de286d992
>>> ("drm/msm/dp: Support up to 3 DP controllers")
>>>
>>>       A new enum is introduced to document the connection between the
>>>       instances referenced in the dpu_intf_cfg array and the controllers in
>>>       the DP driver and sc7180 is updated.
>>>
>>> It sounds like the intent of that commit failed to make a strong enough
>>> connection. Now it needs to match the INTF number as well? I can't
>>> really figure out what is actually wrong, because this patch undoes that
>>> intentional tight coupling. Is the next patch the important part that
>>> flips the order of the two interfaces?
>> The commit bb3de286d992have two problems,
>>
>> 1)  The below sc7280_dp_cfg will not work, if eDP use
>> MSM_DP_CONTROLLER_2 instead of  MSM_DP_CONTROLLER_1
> Why would we use three indices for an soc that only has two indices
> possible? This is not a real problem?

I do not what will happen at future, it may have more dp controller use 
late.

at current soc, below table has only one eDP will not work either.

static const struct msm_dp_config sc7280_dp_cfg = {
          .descs = (const struct msm_dp_desc[]) {
                  [MSM_DP_CONTROLLER_1] = { .io_start = 0x0aea0000,
.connector_type = DRM_MODE_CONNECTOR_eDP, .wide_bus_en = true },

          .num_descs = 1,
};

>
>> since it have num_descs =2 but eDP is at index 2 (CONTROLLER_2) which
>> never be reached.
>>
>> static const struct msm_dp_config sc7280_dp_cfg = {
>>           .descs = (const struct msm_dp_desc[]) {
>>                   [MSM_DP_CONTROLLER_2] = { .io_start = 0x0aea0000,
>> .connector_type = DRM_MODE_CONNECTOR_eDP, .wide_bus_en = true },
>>                   [MSM_DP_CONTROLLER_0] = { .io_start = 0x0ae90000,
>> .connector_type = DRM_MODE_CONNECTOR_DisplayPort, .wide_bus_en = true },
>>           },
>>           .num_descs = 2,
>> };
>>
>> 2)  DP always has index of 0 (dp->id = 0) and the first one to call
>> msm_dp_modeset_init(). This make DP always place at head of bridge chain.
> What does this mean? Are you talking about the list of bridges in drm
> core, i.e. 'bridge_list'?
yes,
>
>> At next patch eDP must be placed at head of bridge chain to fix eDP
>> corruption issue. This is the purpose of this patch. I will revise the
>> commit text.
>>
> Wouldn't that be "broken" again if we decided to change drm_bridge_add()
> to add to the list head instead of list tail? Or if somehow
> msm_dp_modeset_init() was called in a different order so that the DP
> bridge was added before the eDP bridge?

we have no control of drm_bridge_add().

Since drm perform screen update following bridge chain sequentially, we 
have to make sure primary always update first.