On Wed, Sep 23, 2020 at 12:31 PM Ville Syrjälä ville.syrjala@linux.intel.com wrote:

On Tue, Sep 22, 2020 at 08:18:34PM +0200, Daniel Vetter wrote:

...

When doing an atomic modeset with ALLOW_MODESET drivers are allowed to pull in arbitrary other resources, including CRTCs (e.g. when reconfiguring global resources).

But in nonblocking mode userspace has then no idea this happened, which can lead to spurious EBUSY calls, both:

• when that other CRTC is currently busy doing a page_flip the ALLOW_MODESET commit can fail with an EBUSY
• on the other CRTC a normal atomic flip can fail with EBUSY because of the additional commit inserted by the kernel without userspace's knowledge

For blocking commits this isn't a problem, because everyone else will just block until all the CRTC are reconfigured. Only thing userspace can notice is the dropped frames without any reason for why frames got dropped.

Consensus is that we need new uapi to handle this properly, but no one has any idea what exactly the new uapi should look like. Since this has been shipping for years already compositors need to deal no matter what, so as a first step just try to enforce this across drivers better with some checks.

v2: Add comments and a WARN_ON to enforce this only when allowed - we don't want to silently convert page flips into blocking plane updates just because the driver is buggy.

v3: Fix inverted WARN_ON (Pekka).

v4: Drop the uapi changes, only add a WARN_ON for now to enforce some rules for drivers.

References: https://lists.freedesktop.org/archives/dri-devel/2018-July/182281.html Bugzilla: https://gitlab.freedesktop.org/wayland/weston/issues/24#note_9568 Cc: Daniel Stone daniel@fooishbar.org Cc: Pekka Paalanen pekka.paalanen@collabora.co.uk Cc: Simon Ser contact@emersion.fr Cc: stable@vger.kernel.org Cc: Ville Syrjälä ville.syrjala@linux.intel.com Signed-off-by: Daniel Vetter daniel.vetter@intel.com

---

drivers/gpu/drm/drm_atomic.c | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+)

diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c index 58527f151984..ef106e7153a6 100644 --- a/drivers/gpu/drm/drm_atomic.c +++ b/drivers/gpu/drm/drm_atomic.c @@ -281,6 +281,10 @@ 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

@@ -1262,10 +1266,15 @@ 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, old_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) {
  

@@ -1313,6 +1322,24 @@ int drm_atomic_check_only(struct drm_atomic_state *state) } }

• for_each_new_crtc_in_state(state, crtc, old_crtc_state, i)
  

Inconsistent old vs. new.

Will fix, but also doesn't matter since I don't care about the state, just that it's in there.

...

•         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);
  

• }
  

I think this means pretty much all non-pageflip commits will have to have allow_modeset==true on i915 or else we just can't guarantee that we can anything (due to sagv and/or cdclk mainly).

I guess not enough machines with multiple outputs in the shards.

Also a bit baffled that CI didn't hit this. I think it should be totally possible to hit this now. To avoid that I guess we'd just need to make intel_atomic_serialize_global_state() fail if it has to add any new crtcs when allow_modeset==false. Hopefully there aren't many other places that add crtcs to the state without forcing a modeset on them.

Oh we don't do that? That feels like a pretty bad bug ... Wacking random other crtc without allow_modeset is pretty nasty. -Daniel

...

• return 0;
  

} EXPORT_SYMBOL(drm_atomic_check_only); -- 2.28.0

-- Ville Syrjälä Intel