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

Tue May 12 16:19:39 UTC 2020

On Tue, 2020-05-12 at 06:17 +0000, Lee, Shawn C wrote:
> On Thu, May 11, 2020, Ville Syrjälä wrote:
> > On Thu, May 07, 2020 at 06:46:17PM -0400, 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 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.
> > > > > > 
> > > > > > 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 | 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.
> > 
> > We have at least one full_pbn==0 issue in ci:
> > <4>[    5.061345] WARNING: CPU: 0 PID: 376 at
> > drivers/gpu/drm/drm_dp_mst_topology.c:4889
> > drm_dp_mst_atomic_check_mstb_bw_limit+0x193/0x1c0
> > 
> > https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_8460/fi-kbl-7560u/boot0.txt
> > 

Oh yikes-yeah, it's definitely reporting 0 PBN, that's a new one. Anyway-I'll
take a look at this patch sometime today. I'd like to look more into that hub
though, I'm almost certain that full_pbn = 0 is against the MST spec, but the
hub does seem to be reporting that:

<7>[    5.002279] [drm:drm_dp_send_enum_path_resources] enum path resources 3:
0 0

Since hubs will set full_pbn based on the current downstream link training
settings for the mst port though, I wonder if maybe this hub is just failing
to do link training downstream or failing to detect a display downstream?
Probably would be useful to know what kind of a hub this is, and what kind of
displays are hooked up to it. Also, having drm DP enabled in those logs would
be quite helpful as well

> > Dunno if that's the MST device reporting zero or due to some other
> > bug, but at least CI is currently borked on that machine.
> 
> Thanks for sharing! It will be good to keep checking full_pbn to avoid
> unexpected issue.
> BTW, I just upload patch v2. Could you please share your comments about
> that?
> 
> Best regards,
> Shawn
> 
-- 
Cheers,
	Lyude Paul (she/her)
	Associate Software Engineer at Red Hat