Re: [PATCH v5] drm/msm/dp: force link training for display resolution change

[Kuogee Hsieh]

On 6/14/2022 1:38 AM, Stephen Boyd wrote:
> Quoting Kuogee Hsieh (2022-06-13 14:48:37)
> > During display resolution changes display have to be disabled first
> > followed by display enabling with new resolution. Display disable
> > will turn off both pixel clock and main link clock so that main link
> > have to be re-trained during display enable to have new video stream
> > flow again. At current implementation, display enable function manually
> > kicks up irq_hpd_handle which will read panel link status and start link
> > training if link status is not in sync state. However, there is rare
> > case that a particular panel links status keep staying in sync for
> > some period of time after main link had been shut down previously at
> > display disabled. Main link retraining will not be executed by
> > irq_hdp_handle() if the link status read from panel shows it is in
> > sync state. If this was happen, then video stream of newer display
> > resolution will fail to be transmitted to panel due to main link is
> > not in sync between host and panel. This patch force main link always
> > be retrained during display enable procedure to prevent this rare
> > failed case from happening. Also this implementation are more
> > efficient than manual kicking off irq_hpd_handle function.
> How is resolution change different from disabling and enabling the
> display? The commit text talks about resolution changes, but the code
> doesn't compare resolutions from before and after to know when to
> retrain the link. Can the code be made to actually do what the commit
> text says? It would be clearer if the code looked for actual resolution
> changes instead of hooking the dp_bridge_enable() function.
>
> > Changes in v2:
> > -- set force_link_train flag on DP only (is_edp == false)
> >
> > Changes in v3:
> > -- revise commit  text
> > -- add Fixes tag
> >
> > Changes in v4:
> > -- revise commit  text
> >
> > Changes in v5:
> > -- fix spelling at commit text
> >
> > Fixes: 62671d2ef24b ("drm/msm/dp: fixes wrong connection state caused by failure of link train")
> > Signed-off-by: Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/msm/dp/dp_ctrl.c    |  6 +++---
> >   drivers/gpu/drm/msm/dp/dp_ctrl.h    |  2 +-
> >   drivers/gpu/drm/msm/dp/dp_display.c | 15 ++++++++-------
> >   3 files changed, 12 insertions(+), 11 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > index af7a80c..bea93eb 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > @@ -1551,7 +1551,7 @@ static int dp_ctrl_process_phy_test_request(struct dp_ctrl_private *ctrl)
> >
> >          ret = dp_ctrl_on_link(&ctrl->dp_ctrl);
> >          if (!ret)
> > -               ret = dp_ctrl_on_stream(&ctrl->dp_ctrl);
> > +               ret = dp_ctrl_on_stream(&ctrl->dp_ctrl, false);
> Does this even matter if it's true or false? The 'sink_request' has
> DP_TEST_LINK_PHY_TEST_PATTERN set from what I can tell, and then
> dp_ctrl_on_stream() bails out before calling dp_ctrl_link_retrain()
> anyway. It would be nice if we could split dp_ctrl_on_stream() so that
> the part after the check for the sink request is a different function
> that is called by dp_display.c and then this code can call the 'prepare'
> function that does the first part. Then we can ignore the testing path
> in the code, and possibly remove the conditional in dp_ctrl_on_stream()?
>
> >          else
> >                  DRM_ERROR("failed to enable DP link controller\n");
> >
> > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> > index c388323..370348d 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_display.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> > @@ -872,7 +872,7 @@ static int dp_display_enable(struct dp_display_private *dp, u32 data)
> >                  return 0;
> >          }
> >
> > -       rc = dp_ctrl_on_stream(dp->ctrl);
> > +       rc = dp_ctrl_on_stream(dp->ctrl, data);
> >          if (!rc)
> >                  dp_display->power_on = true;
> >
> > @@ -1654,6 +1654,7 @@ void dp_bridge_enable(struct drm_bridge *drm_bridge)
> >          int rc = 0;
> >          struct dp_display_private *dp_display;
> >          u32 state;
> > +       bool force_link_train = false;
> >
> >          dp_display = container_of(dp, struct dp_display_private, dp_display);
> >          if (!dp_display->dp_mode.drm_mode.clock) {
> > @@ -1688,10 +1689,14 @@ void dp_bridge_enable(struct drm_bridge *drm_bridge)
> >
> >          state =  dp_display->hpd_state;
> >
> > -       if (state == ST_DISPLAY_OFF)
> > +       if (state == ST_DISPLAY_OFF) {
> >                  dp_display_host_phy_init(dp_display);
> >
> > -       dp_display_enable(dp_display, 0);
> > +               if (!dp->is_edp)
> I didn't see any answer to my question about why edp is special on v4.
> Can you at least add a comment to the code about why edp doesn't need to
> unconditionally retrain, but DP does?

Sorry, missed this one.

This is my mistake, both DP and eDP are same. will remove is_edp flag 
checking.

> > +                       force_link_train = true;
> > +       }
> > +
> > +       dp_display_enable(dp_display, force_link_train);
> >
> >          rc = dp_display_post_enable(dp);
> >          if (rc) {