[Intel-gfx] [PATCH v3] drm/i915/mst: filter out the display mode exceed sink's capability

[Lyude Paul]

On Fri, 2020-05-22 at 14:35 -0400, Lyude Paul wrote:
> 
> On Tue, 2020-05-19 at 11:56 +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 at 60fps. DP 1.2 can support 4K at 60fps
> > because it did not surpass DP physical bandwidth limitation.
> > Do modeset to 4k at 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.
> > 
> > v2: Using mgr->base.lock to protect full_pbn.
> > v3: Add ctx lock.
> > 
> > Cc: Manasi Navare <manasi.d.navare at intel.com>
> > Cc: Jani Nikula <jani.nikula at linux.intel.com>
> > Cc: Ville Syrjala <ville.syrjala at linux.intel.com>
> > Cc: Cooper Chiou <cooper.chiou at intel.com>
> > Cc: Lyude Paul <lyude at redhat.com>
> > Signed-off-by: Lee Shawn C <shawn.c.lee at intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp_mst.c | 30 ++++++++++++++++++++-
> >  1 file changed, 29 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 74559379384a..6b6f7eef5b68 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > @@ -611,6 +611,32 @@ 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;
> > +	struct drm_modeset_acquire_ctx ctx;
> > +	bool ret = false;
> > +
> > +	if (!mgr)
> > +		return ret;
> > +
> > +	drm_modeset_acquire_init(&ctx, 0);
> > +
> > +	drm_modeset_lock(&mgr->base.lock, &ctx);
> > +	if (port->full_pbn)
> > +		ret = (drm_dp_calc_pbn_mode(clock, bpp, false) > port-
> > > full_pbn);
> > +	drm_modeset_unlock(&mgr->base.lock);
> > +
> > +	drm_modeset_drop_locks(&ctx);
> > +	drm_modeset_acquire_fini(&ctx);
> > +
> > +	return ret;
> > +}
> 
> Sorry to do this, but this still isn't right :s, and will likely end up
> deadlocking. I'm going to write up a fixed version of this patch and add you
> in
> the Co-developed-by: tag, shouldn't take too long for me to do so I can send
> it
> out today

Completely forgot to actually explain what was wrong here - this creates an
acquisition context that would be separate from the caller's lock acquisition
context. So, drm_helper_probe_single_connector_modes() would grab
connection_mutex before calling the hook, and since it's not using an
acquisition context we could potentially deadlock when locking &mgr->base.lock
if another thread locked &mgr->base.lock before connection_mutex. If we add a
lock acquisition context to drm_helper_probe_single_connector_modes and use that
to grab &mgr->base.lock, the ww-mutex code can detect the deadlock and give us a
chance to back off and try again.

anyway-i'll send out the new version of the patch asap

> > +
> >  static enum drm_mode_status
> >  intel_dp_mst_mode_valid(struct drm_connector *connector,
> >  			struct drm_display_mode *mode)
> > @@ -633,7 +659,9 @@ 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;
> >