[PATCH] drm: document and enforce rules around "spurious" EBUSY from atomic_commit

[Daniel Stone]

Hi,

On Tue, 22 Sep 2020 at 19:18, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> +       for_each_new_crtc_in_state(state, crtc, old_crtc_state, i)
> +               affected_crtc |= drm_crtc_mask(crtc);
> +
> +       /*
> +        * For commits that allow modesets drivers can add other CRTCs to the
> +        * atomic commit, e.g. when they need to reallocate global resources.
> +        * This can cause spurious EBUSY, which robs compositors of a very
> +        * effective sanity check for their drawing loop. Therefor only allow
> +        * this for modeset commits.
> +        *
> +        * FIXME: Should add affected_crtc mask to the ATOMIC IOCTL as an output
> +        * so compositors know what's going on.
> +        */
> +       if (affected_crtc != requested_crtc) {
> +               /* adding other CRTC is only allowed for modeset commits */
> +               WARN_ON(!state->allow_modeset);
> +       }
> +

Can we please add a DRM_DEBUG() here, regardless of the WARN_ON(), to
let people know what's happening?

With that, R-b me.

Cheers,
Daniel