[Intel-gfx] [PATCH] Revert "drm/atomic: document and enforce rules around "spurious" EBUSY"

[Daniel Vetter]

On Tue, Feb 09, 2021 at 04:14:01PM -0800, Manasi Navare wrote:
> This reverts commit fb6473a48b635c55d04eb94e579eede52ef39550.
> 
> These additional checks added to avoid EBUSY give unnecessary WARN_ON
> in case of big joiner used in i915 in which case even if the modeset
> is requested on a single pipe, internally another consecutive
> pipe is stolen and used to drive half of the transcoder timings.
> So in this case it is expected that requested crtc and affected crtcs
> do not match. Hence the added WARN ON becomes irrelevant.
> But there is no easy solution to get the bigjoiner information
> here at drm level. So for now revert this until we work out
> a better solution.
> 
> Cc: Ville Syrjala <ville.syrjala at linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter at intel.com>
> Signed-off-by: Manasi Navare <manasi.d.navare at intel.com>

Nope. We can maybe rework this so that i915 can do stuff under the hood,
but wrt uapi this was the thing we discussed with compositors. Without
such a guarantee atomic is defacto broken from a compositor pov.

This WARN_ON is not unecessary, compositor people really do not want the
kernel to throw around spurious EBUSY they have no visibility into.

Please also cc all the compositor people from my original patch if you
change anything in this area.
-Daniel


> ---
>  drivers/gpu/drm/drm_atomic.c | 29 -----------------------------
>  1 file changed, 29 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index b1efa9322be2..48b2262d69f6 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -320,10 +320,6 @@ EXPORT_SYMBOL(__drm_atomic_state_free);
>   * needed. It will also grab the relevant CRTC lock to make sure that the state
>   * is consistent.
>   *
> - * WARNING: Drivers may only add new CRTC states to a @state if
> - * drm_atomic_state.allow_modeset is set, or if it's a driver-internal commit
> - * not created by userspace through an IOCTL call.
> - *
>   * Returns:
>   *
>   * Either the allocated state or the error code encoded into the pointer. When
> @@ -1306,15 +1302,10 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
>  	struct drm_crtc_state *new_crtc_state;
>  	struct drm_connector *conn;
>  	struct drm_connector_state *conn_state;
> -	unsigned requested_crtc = 0;
> -	unsigned affected_crtc = 0;
>  	int i, ret = 0;
>  
>  	DRM_DEBUG_ATOMIC("checking %p\n", state);
>  
> -	for_each_new_crtc_in_state(state, crtc, new_crtc_state, i)
> -		requested_crtc |= drm_crtc_mask(crtc);
> -
>  	for_each_oldnew_plane_in_state(state, plane, old_plane_state, new_plane_state, i) {
>  		ret = drm_atomic_plane_check(old_plane_state, new_plane_state);
>  		if (ret) {
> @@ -1362,26 +1353,6 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
>  		}
>  	}
>  
> -	for_each_new_crtc_in_state(state, crtc, new_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
> -	 * drivers to add unrelated CRTC states 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) {
> -		DRM_DEBUG_ATOMIC("driver added CRTC to commit: requested 0x%x, affected 0x%0x\n",
> -				 requested_crtc, affected_crtc);
> -		WARN(!state->allow_modeset, "adding CRTC not allowed without modesets: requested 0x%x, affected 0x%0x\n",
> -		     requested_crtc, affected_crtc);
> -	}
> -
>  	return 0;
>  }
>  EXPORT_SYMBOL(drm_atomic_check_only);
> -- 
> 2.19.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch