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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Date:   Fri, 24 Jun 2022 15:19:11 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Kuogee Hsieh <quic_khsieh@...cinc.com>, 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

Quoting Kuogee Hsieh (2022-06-24 14:49:57)
>
> 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,

So the MSM_DP_CONTROLLER_* number needs to match what exactly?

>
> >
> >> 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,

I changed the drm_bridge_add() API and that doesn't make any difference.
The corruption is still seen. That would imply it is not the order of
the list of bridges.

---8<---
diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index e275b4ca344b..e3518101b65e 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -165,7 +165,7 @@ void drm_bridge_add(struct drm_bridge *bridge)
 	mutex_init(&bridge->hpd_mutex);

 	mutex_lock(&bridge_lock);
-	list_add_tail(&bridge->list, &bridge_list);
+	list_add(&bridge->list, &bridge_list);
 	mutex_unlock(&bridge_lock);
 }
 EXPORT_SYMBOL(drm_bridge_add);

> >
> >> 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.
>