Re: [PATCH v2] drm/nouveau: Only recalculate PBN/VCPI on mode/connector changes

[Ben Skeggs <skeggsb@gmail.com>]

On Fri, 9 Aug 2019 at 10:53, Lyude Paul <lyude@redhat.com> 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.
>
> Changes since v1:
> * Fix accidental reversal of clock and bpp arguments in
>   drm_dp_calc_pbn_mode() - William Lewis
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Reported-by: Bohdan Milar <bmilar@redhat.com>
> Tested-by: Bohdan Milar <bmilar@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@redhat.com>
> Cc: Ben Skeggs <bskeggs@redhat.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: David Airlie <airlied@redhat.com>
> Cc: Jerry Zuo <Jerry.Zuo@amd.com>
> Cc: Harry Wentland <harry.wentland@amd.com>
> Cc: Juston Li <juston.li@intel.com>
> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> Cc: Karol Herbst <karolherbst@gmail.com>
> Cc: Ilia Mirkin <imirkin@alum.mit.edu>
> Cc: <stable@vger.kernel.org> # v5.1+
Acked-by: Ben Skeggs <bskeggs@redhat.com>

> ---
>  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..5c36c75232e6 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);
> +       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(clock, bpp);
> +               }
>
> -       if (crtc_state->mode_changed) {
>                 slots = drm_dp_atomic_find_vcpi_slots(state, &mstm->mgr,
>                                                       mstc->port,
>                                                       asyh->dp.pbn);
> --
> 2.21.0
>
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