Re: [Intel-gfx] [PATCH 2/6] drm/i915: Fix bw atomic check when switching between SAGV vs. no SAGV

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Feb 14, 2022 at 12:05:36PM +0200, Lisovskiy, Stanislav wrote:
> On Mon, Feb 14, 2022 at 11:18:07AM +0200, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > If the only thing that is changing is SAGV vs. no SAGV but
> > the number of active planes and the total data rates end up
> > unchanged we currently bail out of intel_bw_atomic_check()
> > early and forget to actually compute the new WGV point
> > mask and thus won't actually enable/disable SAGV as requested.
> > This ends up poorly if we end up running with SAGV enabled
> > when we shouldn't. Usually ends up in underruns.
> > To fix this let's go through the QGV point mask computation
> > if anyone else already added the bw state for us.
> 
> Haven't been looking this in a while. Despite we have been
> looking like few revisions together still some bugs :(
> 
> I thought SAGV vs No SAGV can't change if active planes 
> or data rate didn't change? Because it means we probably
> still have same ddb allocations, which means SAGV state
> will just stay the same.

SAGV can change due to watermarks/ddb allocations. The easiest
way to trip this up is to try to use the async flip wm0/ddb 
optimization. That immediately forgets to turn off SAGV and
we get underruns, whcih is how I noticed this. And I don't
immediately see any easy proof that this couldn't also happen
due to some other plane changes.

> 
> Stan
> 
> > 
> > Cc: stable@vger.kernel.org
> > Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> > Fixes: 20f505f22531 ("drm/i915: Restrict qgv points which don't have enough bandwidth.")
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_bw.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_bw.c b/drivers/gpu/drm/i915/display/intel_bw.c
> > index 23aa8e06de18..d72ccee7d53b 100644
> > --- a/drivers/gpu/drm/i915/display/intel_bw.c
> > +++ b/drivers/gpu/drm/i915/display/intel_bw.c
> > @@ -846,6 +846,13 @@ int intel_bw_atomic_check(struct intel_atomic_state *state)
> >  	if (num_psf_gv_points > 0)
> >  		mask |= REG_GENMASK(num_psf_gv_points - 1, 0) << ADLS_PSF_PT_SHIFT;
> >  
> > +	/*
> > +	 * If we already have the bw state then recompute everything
> > +	 * even if pipe data_rate / active_planes didn't change.
> > +	 * Other things (such as SAGV) may have changed.
> > +	 */
> > +	new_bw_state = intel_atomic_get_new_bw_state(state);
> > +
> >  	for_each_oldnew_intel_crtc_in_state(state, crtc, old_crtc_state,
> >  					    new_crtc_state, i) {
> >  		unsigned int old_data_rate =
> > -- 
> > 2.34.1
> > 

-- 
Ville Syrjälä
Intel