Re: [PATCH] drm: remove drm_bridge_hpd_disable() from drm_bridge_connector_destroy()

[Abhinav Kumar]

Hi Laurent

On 9/19/2023 11:12 AM, Laurent Pinchart wrote:
> Hi Abhinav,
>
> Thank you for the patch.
>
> On Tue, Sep 19, 2023 at 10:48:12AM -0700, Abhinav Kumar wrote:
> > drm_bridge_hpd_enable()/drm_bridge_hpd_disable() callbacks call into
> > the respective driver's hpd_enable()/hpd_disable() ops. These ops control
> > the HPD enable/disable logic which in some cases like MSM can be a
> > dedicate hardware block to control the HPD.
> >
> > During probe_defer cases, a connector can be initialized and then later
> > destroyed till the probe is retried. During connector destroy in these
> > cases, the hpd_disable() callback gets called without a corresponding
> > hpd_enable() leading to an unbalanced state potentially causing even
> > a crash.
> >
> > This can be avoided by the respective drivers maintaining their own
> > state logic to ensure that a hpd_disable() without a corresponding
> > hpd_enable() just returns without doing anything.
> >
> > However, to have a generic fix it would be better to avoid the
> > hpd_disable() callback from the connector destroy path and let
> > the hpd_enable() / hpd_disable() balance be maintained by the
> > corresponding drm_bridge_connector_enable_hpd() /
> > drm_bridge_connector_disable_hpd() APIs which should get called by
> > drm_kms_helper_disable_hpd().
>
> The change makes sense to me, but I'm a bit worried this could introduce
> a regression by leaving HPD enabled in some cases.
>
> I agree that bridges shouldn't track the HPD state, it should be tracked
> by the core and the .enable_hpd() and .disable_hpd() operations should
> be balanced. Their documentation, however, doesn't clearly state this,
> and the documentation of the callers of these operations is also fairly
> unclear.
>
> Could you perhaps try to improve the documentation ? With that,

Yes, sure, Let me upload another patch to improve the documentation of 
.enable_hpd(), .disable_hpd() and its callers.

> Reviewed-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
>
> for this patch.

Thanks

Abhinav

> > changes in v2:
> > 	- minor change in commit text (Dmitry)
> >
> > Signed-off-by: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx>
> > Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/drm_bridge_connector.c | 6 ------
> >   1 file changed, 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_bridge_connector.c b/drivers/gpu/drm/drm_bridge_connector.c
> > index 1da93d5a1f61..c4dba39acfd8 100644
> > --- a/drivers/gpu/drm/drm_bridge_connector.c
> > +++ b/drivers/gpu/drm/drm_bridge_connector.c
> > @@ -187,12 +187,6 @@ static void drm_bridge_connector_destroy(struct drm_connector *connector)
> >   	struct drm_bridge_connector *bridge_connector =
> >   		to_drm_bridge_connector(connector);
> >   
> > -	if (bridge_connector->bridge_hpd) {
> > -		struct drm_bridge *hpd = bridge_connector->bridge_hpd;
> > -
> > -		drm_bridge_hpd_disable(hpd);
> > -	}
> > -
> >   	drm_connector_unregister(connector);
> >   	drm_connector_cleanup(connector);
> >