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, 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@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.
>
>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
>
>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
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