Re: [Intel-gfx] [PATCH] drm/i915/mst: filter out the display mode exceed sink's capability

[Lyude Paul <lyude@redhat.com>]

On Thu, 2020-04-30 at 02:37 +0000, Lee, Shawn C wrote:
> On Sat, 2020-04-25, Lyude Paul wrote:
> > Hi! Sorry this took me a little while to get back to, I had a couple of
> > MST regressions that I had to look into
> > 
> > On Sat, 2020-04-18 at 05:24 +0800, Lee Shawn C wrote:
> > > So far, max dot clock rate for MST mode rely on physcial bandwidth 
> > > limitation. It would caused compatibility issue if source display 
> > > resolution exceed MST hub output ability.
> > > 
> > > For example, source DUT had DP 1.2 output capability.
> > > And MST docking just support HDMI 1.4 spec. When a HDMI 2.0 monitor 
> > > connected. Source would retrieve EDID from external and get max 
> > > resolution 4k@60fps. DP 1.2 can support 4K@60fps because it did not 
> > > surpass DP physical bandwidth limitation.
> > > Do modeset to 4k@60fps, source output display data but MST docking 
> > > can't output HDMI properly due to this resolution already over HDMI 
> > > 1.4 spec.
> > > 
> > > Refer to commit <fcf463807596> ("drm/dp_mst: Use full_pbn instead of 
> > > available_pbn for bandwidth checks").
> > > Source driver should refer to full_pbn to evaluate sink output 
> > > capability. And filter out the resolution surpass sink output 
> > > limitation.
> > > 
> > > Cc: Manasi Navare <manasi.d.navare@intel.com>
> > > Cc: Jani Nikula <jani.nikula@linux.intel.com>
> > > Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
> > > Cc: Cooper Chiou <cooper.chiou@intel.com>
> > > Cc: Lyude Paul <lyude@redhat.com>
> > > Signed-off-by: Lee Shawn C <shawn.c.lee@intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/display/intel_dp_mst.c | 24 
> > > ++++++++++++++++++++-
> > >  1 file changed, 23 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > > b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > > index 61605eb8c2af..68697f463dab 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > > @@ -609,6 +609,26 @@ static int intel_dp_mst_get_modes(struct 
> > > drm_connector
> > > *connector)
> > >  	return intel_dp_mst_get_ddc_modes(connector);
> > >  }
> > >  
> > > +static bool
> > > +intel_dp_mst_mode_clock_exceed_pbn_bandwidth(struct drm_connector
> > > *connector, int clock, int bpp)
> > > +{
> > > +	struct intel_connector *intel_connector =
> > > to_intel_connector(connector);
> > > +	struct intel_dp *intel_dp = intel_connector->mst_port;
> > > +	struct drm_dp_mst_topology_mgr *mgr = &intel_dp->mst_mgr;
> > > +	struct drm_dp_mst_port *port = (to_intel_connector(connector))->port;
> > > +	bool ret = false;
> > > +
> > > +	if (!mgr)
> > > +		return ret;
> > > +
> > > +	mutex_lock(&mgr->lock);
> > 
> > This isn't the right lock for this - mgr->lock protects the topology
> > layout (e.g. drm_dp_mst_port.mstb, drm_dp_mst_port.next, and
> > drm_dp_mst_branch.ports). port->full_pbn is protected under
> > &drm_dp_mst_topology_mgr.base.lock (not drm_dp_mst_topology_mgr.lock), so
> > you need to first add a version of .mode_valid() to struct
> > drm_connector_helper_funcs that accepts a drm_modeset_acquire_ctx so you
> > can use that to safely grab &drm_dp_mst_topology_mgr.base.lock.
> > 
> 
> Thanks for comments! I will correct the lock to protect port->full_pbn.
> 
> > > +	if (port->full_pbn)
> > 
> > Also - there's no reason to check if (port->full_pbn) here, so long as
> > you're holding the right locks this should always be populated by the time
> > we create the drm_connector for the MST port (if it's not, that's a bug
> > that needs to be fixed).
> > 
> 
> Just want to make sure full_pbn value is greater than zero. As you mention
> in another patch, it's hard to predict sink report full or available PBN
> properly.

Sorry this took me a little while to respond to, crunch time came up at work…
Anyway-have you seen issues with full_pbn reporting on hubs? I've seen plenty
of problems with available_pbn reporting, but the reason we switched over to
full_pbn in the first place is because that seemed to be the one hubs reported
reliably. We actually make the assumption full_pbn is always > 0 if
something's connected in some places in the MST helpers, so if we've got cases
of full_pbn lying as well on some hubs we might want to fix that.

> 
> > > +		ret = (drm_dp_calc_pbn_mode(clock, bpp, false) > port-
> > > > full_pbn);
> > 
> > Finally - I'd say we should probably have a separate patch to add a helper
> > for calculating the PBN and checking it against port->full_pbn. Maybe
> > something that looks like this:
> > 
> > int drm_dp_mst_mode_valid(struct drm_dp_mst_port *port, struct
> > drm_modeset_acquire_ctx *ctx, int clock, int bpp) {
> > 	int ret = MODE_VALID;
> > 	/* TODO: DSC support */
> > 
> > 	/* ...try grabbing locks here...*/
> > 	if (drm_dp_calc_pbn_mode(clock, bpp, false) > port->full_pbn)
> > 		ret = MODE_CLOCK_HIGH;
> > 
> > 	return ret;
> > }
> > 
> > That way we might be able to add some checks for DSC capable connectors
> > later once I've migrated most of the DSC code from amdgpu into the MST
> > helpers
> 
> This sounds good. DRM driver provide a public function call for different
> vendor to check the current mode exceed PBN's limitation or not.
> 
> > > +	mutex_unlock(&mgr->lock);
> > > +
> > > +	return ret;
> > > +}
> > > +
> > >  static enum drm_mode_status
> > >  intel_dp_mst_mode_valid(struct drm_connector *connector,
> > >  			struct drm_display_mode *mode)
> > > @@ -631,7 +651,9 @@ intel_dp_mst_mode_valid(struct drm_connector
> > > *connector,
> > >  	max_rate = intel_dp_max_data_rate(max_link_clock, max_lanes);
> > >  	mode_rate = intel_dp_link_required(mode->clock, 18);
> > >  
> > > -	/* TODO - validate mode against available PBN for link */
> > > +	if (intel_dp_mst_mode_clock_exceed_pbn_bandwidth(connector, mode-
> > > > clock, 24))
> > > +		return MODE_CLOCK_HIGH;
> > > +
> > >  	if (mode->clock < 10000)
> > >  		return MODE_CLOCK_LOW;
> > >  
> > --
> > Cheers,
> > 	Lyude Paul (she/her)
> > 	Associate Software Engineer at Red Hat
> > 
> 
> Best regards,
> Shawn
> 
-- 
Cheers,
	Lyude Paul (she/her)
	Associate Software Engineer at Red Hat

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