Hi Daniel,
Just got bitten by this warning when trying to do some refactoring in
amdgpu for trying to get rid of the DRM private object we use for our DC
state.
From a userspace perspective I understand that we want to avoid judder,
-EBUSY and other issues affecting the compositor from kernel having to
drag these CRTCs (or their planes) into the atomic state.
For bandwidth validation we need to understand the state of all CRTCs
and planes in use. Existing driver code maintains this as part of a
global state object in a DRM private atomic state. We have stalls in
atomic check (bad) to avoid freeing this state or modifying it at the
wrong times which avoid hitting this warning but essentially cause the
same judder issue.
While most hardware has independent pipes, I think almost all hardware
ends up having the memory interface/bandwidth as a global shared
resource that software state can't really abstract around.
There are cases where we know that there will be no (or minimal) impact
to the overall memory requirements for particular DRM updates. Our
validation already "over-allocates" for common display changes - page
flips, some format changes, cursor enable/disable. But for most cases
outside of that we do want to pull in _all_ the CRTCs and planes.
On our HW you won't get a blankout unless you're actually modifying a
stream timing itself so I think the ALLOW_MODESET flag is overkill here.
Rejecting the commit when the flag isn't set also ends up breaking
userspace in the process since it expects commits like pageflips between
different tiling modes to succeed with the legacy IOCTLs.
Any ideas about this? I missed the IRC discussion regarding this before
so I'm not sure if we have any alternatives that were dropped in favor
of this.
Regards,
Nicholas Kazlauskas
On 2020-09-25 4:46 a.m., 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.
v5: Make the WARNING more informative (Daniel)
v6: Add unconditional debug output for compositor hackers to figure
out what's going on when they get an EBUSY (Daniel)
v7: Fix up old/new_crtc_state confusion for real (Pekka/Ville)
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@xxxxxxxxxxxxx>
Cc: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxxxx>
Cc: Simon Ser <contact@xxxxxxxxxxx>
Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
drivers/gpu/drm/drm_atomic.c | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)
diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 58527f151984..aac9122f1da2 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, 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) {
@@ -1313,6 +1322,26 @@ 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);
