Re: [PATCH] drm/i915/dp/mst: Validate modes against the available link bandwidth

[Jim Bride <jim.bride@linux.intel.com>]

On Tue, Sep 06, 2016 at 06:39:12PM +0000, Srivatsa, Anusha wrote:
> Sending to the list again since Ville's review comment  didn't hit the mailing list last time.
> 
> Regards,
> Anusha
> 
> -----Original Message-----
> From: Ville Syrjälä [mailto:ville.syrjala@linux.intel.com] 
> Sent: Monday, September 5, 2016 4:39 AM
> To: Srivatsa, Anusha <anusha.srivatsa@intel.com>
> Cc: intel-gfx@lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH] drm/i915/dp/mst: Validate modes against the available link bandwidth
> 
> On Thu, Aug 18, 2016 at 05:11:31PM -0700, Anusha Srivatsa wrote:
> > Change intel_dp_mst_mode_valid() to use available link bandwidth 
> > rather than the link's maximum supported bandwidth to evaluate whether 
> > modes are legal for the current configuration. This takes into account 
> > the fact that link bandwidth may already be dedicated to other virtual 
> > channels.
> 
> We can't do this. Results of mode_valid() aren't supposed to change like this depending on what else is happening in the system. The only thing mode_valid() tells us is whether it's possible to use the mode under *some* circumstances. Only if the mode can't be used under *any* circumstances should it be filtered out.
> 
> If we wanted to change this, we'd at the very least have to synthesize hotplug uevents whenever the conditions changed. But doing that would be a fairly big behavioural change, so I'm not sure how people feel about it. It also doesn't really solve the problem since eg. atomic can go directly from 0->n active pipes, so there's no way to know upfront which modes we can pick for each pipe.
> 

I won't dispute that this won't help for all cases, but it does make hotplugs,
at a minimum, more sane.  For that reason alone, I'd like to see this patch
land.  Longer term I think we should look at how we can make user space
and atomic better handle MST (IMHO in multi-modeset operations on MST atomic
should ensure that the aggregate dotclock of the modes being set are less
than the link bandwidth that's configured.)  I think that user space also
needs to be more MST aware and update what's available based on the current
configuration at any given time.  I'm not sure what the precise solution
should look like here, but I'd think that what we want is for user space
to fetch the list of available resolutions based on the current configuration
any time we plug or unplug a device on a DP MST topology.  In an ideal world
it would be nice to have some sort of information about the total link
bandwidth, and how much bandwidth each mode is taking up so that the user
could have a guide to tweaking their setup in such a way to maximize how
they choose to configure their monitors based on the available link
bandwidth.

Jim


> > 
> > Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dp_mst.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c 
> > b/drivers/gpu/drm/i915/intel_dp_mst.c
> > index 629337d..39c58eb 100644
> > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > @@ -352,16 +352,22 @@ static enum drm_mode_status  
> > intel_dp_mst_mode_valid(struct drm_connector *connector,
> >  			struct drm_display_mode *mode)
> >  {
> > -	int max_dotclk = to_i915(connector->dev)->max_dotclk_freq;
> > +	int req_pbn = 0;
> > +	int slots = 0;
> > +	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;
> > +
> > +	req_pbn = drm_dp_calc_pbn_mode(mode->clock, 24);
> > +	slots = drm_dp_find_vcpi_slots(mgr, req_pbn);
> >  
> > -	/* TODO - validate mode against available PBN for link */
> >  	if (mode->clock < 10000)
> >  		return MODE_CLOCK_LOW;
> >  
> >  	if (mode->flags & DRM_MODE_FLAG_DBLCLK)
> >  		return MODE_H_ILLEGAL;
> >  
> > -	if (mode->clock > max_dotclk)
> > +	if (slots == -ENOSPC)
> >  		return MODE_CLOCK_HIGH;
> >  
> >  	return MODE_OK;
> > --
> > 2.7.4
> > 
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> --
> Ville Syrjälä
> Intel OTC
> _______________________________________________
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> Intel-gfx@lists.freedesktop.org
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