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

[Lyude Paul]

On Thu, 2019-08-08 at 20:46 -0400, Lyude Paul wrote:
> 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

FWIW, I've sent out a respin with this fixed already, but I see how this didn't
manage to break anything now. The only time bpp and clock are used in
drm_dp_calc_pbn_mode() is:

kbps = clock * bpp;

So I guess that explains it :)

> 
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