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

["Lee, Shawn C" <shawn.c.lee@intel.com>]

On Thu, 2020-05-07, Lyude Paul wrote:
>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.
>

No, we don't see MST hub report full_pbn = 0 so far. Just like what you said.
This is used to guarantee driver always have valid full_pbn for bandwidth calcuation.
If not, driver just bypass the PBN comparison.

>> 
>> > > +		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
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