[Nouveau] [PATCH] drm/nouveau: Only recalculate PBN/VCPI on mode/connector changes

[Lyude Paul]

On Thu, 2019-08-08 at 18:17 +0000, William Lewis wrote:
> On 8/8/19 1:04 PM, Lyude Paul wrote:
> > I -thought- I had fixed this entirely, but it looks like that I didn't
> > test this thoroughly enough as we apparently still make one big mistake
> > with nv50_msto_atomic_check() - we don't handle the following scenario:
> > 
> > * CRTC #1 has n VCPI allocated to it, is attached to connector DP-4
> >    which is attached to encoder #1. enabled=y active=n
> > * CRTC #1 is changed from DP-4 to DP-5, causing:
> >    * DP-4 crtc=#1→NULL (VCPI n→0)
> >    * DP-5 crtc=NULL→#1
> >    * CRTC #1 steals encoder #1 back from DP-4 and gives it to DP-5
> >    * CRTC #1 maintains the same mode as before, just with a different
> >      connector
> > * mode_changed=n connectors_changed=y
> >    (we _SHOULD_ do VCPI 0→n here, but don't)
> > 
> > Once the above scenario is repeated once, we'll attempt freeing VCPI
> > from the connector that we didn't allocate due to the connectors
> > changing, but the mode staying the same. Sigh.
> > 
> > Since nv50_msto_atomic_check() has broken a few times now, let's rethink
> > things a bit to be more careful: limit both VCPI/PBN allocations to
> > mode_changed || connectors_changed, since neither VCPI or PBN should
> > ever need to change outside of routing and mode changes.
> > 
> > Signed-off-by: Lyude Paul <lyude at redhat.com>
> > Reported-by: Bohdan Milar <bmilar at redhat.com>
> > Tested-by: Bohdan Milar <bmilar at redhat.com>
> > Fixes: 232c9eec417a ("drm/nouveau: Use atomic VCPI helpers for MST")
> > References: 412e85b60531 ("drm/nouveau: Only release VCPI slots on mode
> > changes")
> > Cc: Lyude Paul <lyude at redhat.com>
> > Cc: Ben Skeggs <bskeggs at redhat.com>
> > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > Cc: David Airlie <airlied at redhat.com>
> > Cc: Jerry Zuo <Jerry.Zuo at amd.com>
> > Cc: Harry Wentland <harry.wentland at amd.com>
> > Cc: Juston Li <juston.li at intel.com>
> > Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > Cc: Karol Herbst <karolherbst at gmail.com>
> > Cc: Ilia Mirkin <imirkin at alum.mit.edu>
> > Cc: <stable at vger.kernel.org> # v5.1+
> > ---
> >   drivers/gpu/drm/nouveau/dispnv50/disp.c | 22 +++++++++++++---------
> >   1 file changed, 13 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/nouveau/dispnv50/disp.c
> > b/drivers/gpu/drm/nouveau/dispnv50/disp.c
> > index 126703816794..5d23ab8e4917 100644
> > --- a/drivers/gpu/drm/nouveau/dispnv50/disp.c
> > +++ b/drivers/gpu/drm/nouveau/dispnv50/disp.c
> > @@ -771,16 +771,20 @@ nv50_msto_atomic_check(struct drm_encoder *encoder,
> >   	struct nv50_head_atom *asyh = nv50_head_atom(crtc_state);
> >   	int slots;
> >   
> > -	/* When restoring duplicated states, we need to make sure that the
> > -	 * bw remains the same and avoid recalculating it, as the connector's
> > -	 * bpc may have changed after the state was duplicated
> > -	 */
> > -	if (!state->duplicated)
> > -		asyh->dp.pbn =
> > -			drm_dp_calc_pbn_mode(crtc_state->adjusted_mode.clock,
> > -					     connector->display_info.bpc * 3);
> drm_dp_calc_pbn_mode(clock, bpp)
> > +	if (crtc_state->mode_changed || crtc_state->connectors_changed) {
> > +		/*
> > +		 * When restoring duplicated states, we need to make sure that
> > +		 * the bw remains the same and avoid recalculating it, as the
> > +		 * connector's bpc may have changed after the state was
> > +		 * duplicated
> > +		 */
> > +		if (!state->duplicated) {
> > +			const int bpp = connector->display_info.bpc * 3;
> > +			const int clock = crtc_state->adjusted_mode.clock;
> > +
> > +			asyh->dp.pbn = drm_dp_calc_pbn_mode(bpp, clock);
> drm_dp_calc_pbn_mode(bpp, clock)
> > +		}
> >   
> > -	if (crtc_state->mode_changed) {
> >   		slots = drm_dp_atomic_find_vcpi_slots(state, &mstm->mgr,
> >   						      mstc->port,
> >   						      asyh->dp.pbn);
> Which is correct?

!?!?!?!?!?!?
I am actually seriously confused as to how this code did not break when I tested
it, as I certainly reversed the two arguments here by accident.
Looks like we need to add more error checking for drm_dp_calc_pbn_mode.
Thank you for catching this, I'll respin this and send out a new version in a
second

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