[Intel-gfx] [PATCH v2] drm/i915/mst: filter out the display mode exceed sink's capability

[Lyude Paul]

sorry it's been taking me a while to get back to you with these, crunch time at
my work started so i've been really busy over the last week.

On Tue, 2020-05-12 at 07:19 +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.
> 
> 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 74559379384a..6b1864ce3771 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -611,6 +611,26 @@ 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;
> +
> +	drm_modeset_lock(&mgr->base.lock, NULL);
> +	if (port->full_pbn)
> +		ret = (drm_dp_calc_pbn_mode(clock, bpp, false) > port-
> >full_pbn);
> +	drm_modeset_unlock(&mgr->base.lock);

You're so close, but not quite! I'm fairly sure ->mode_valid() gets called with
drm_device.mode_config.connection_mutex already locked, which means that you
need to use the same drm_modeset_acquire context (if there isn't one, you need
to make it) so that you can safely roll back and retry in case there's a
deadlock between drm_dp_mst_topology_mgr.base.lock and
drm_device.mode_config.connection_mutex .

fwiw - I'm sensing some confusion here about ww_mutexes (wouldn't be the first
time, they're unusual). If you haven't already, it's probably worth reading up
on:

https://lwn.net/Articles/548909/

Keep in mind when you pipe down the context by adding a new version of the
->mode_valid() hook you won't want to drop any locks once you're done in this
function. They'll get dropped automatically when the caller of the new
->mode_valid() hook (so, the one where you get the drm_modeset_acquire_ctx from)
calls drm_modeset_drop_locks().

> +
> +	return ret;
> +}
> +
>  static enum drm_mode_status
>  intel_dp_mst_mode_valid(struct drm_connector *connector,
>  			struct drm_display_mode *mode)
> @@ -633,7 +653,9 @@ 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;
>