If we are already in the desired write domain of a set-domain ioctl,
then there is nothing for us to do and we can quickly return back to
userspace, avoiding any lock contention. By recognising that the
write_domain is always a subset of the read_domains, and excluding the
no-op case of requiring 0 read_domains in the ioctl, we can infer if the
current write_domain matches the target read_domains, there is nothing
for us to do.
Secondary aspect of this is that we undo the arbitrary fetching and
potential flushing of all pages for a set-domain(.write=CPU) call on a
fresh object -- which was introduced simply because we do the get-pages
before taking the struct_mutex.
References: 40e62d5d6be8 ("drm/i915: Acquire the backing storage outside of struct_mutex in set-domain")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Cc: Matthew Auld <matthew.william.auld@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_gem.c | 26 +++++++++++++++++++++++---
1 file changed, 23 insertions(+), 3 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 1a684b7e8c09..85a19b906e72 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1484,17 +1484,37 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
if ((write_domain | read_domains) & I915_GEM_GPU_DOMAINS)
return -EINVAL;
- /* Having something in the write domain implies it's in the read
+ /*
+ * Having something in the write domain implies it's in the read
* domain, and only that read domain. Enforce that in the request.
*/
- if (write_domain != 0 && read_domains != write_domain)
+ if (write_domain && read_domains != write_domain)
return -EINVAL;
+ if (!read_domains)
+ return 0;
+
obj = i915_gem_object_lookup(file, args->handle);
if (!obj)
return -ENOENT;
- /* Try to flush the object off the GPU without holding the lock.
+ /*
+ * Already in the desired target write domain? Nothing for us to!
+ *
+ * We apply a little bit of cunning here to catch a broader set of
+ * no-ops. If obj->write_domain is set, we must be in the same
+ * obj->read_domains, and only that domain. Therefore, if that
+ * obj->write_domain matches the request read_domains, we are
+ * already in the same read/write domain and can skip the operation,
+ * without having to further check the requested write_domain.
+ */
+ if (READ_ONCE(obj->write_domain) == read_domains) {
+ err = 0;
+ goto out;
+ }
+
+ /*
+ * Try to flush the object off the GPU without holding the lock.
* We will repeat the flush holding the lock in the normal manner
* to catch cases where we are gazumped.
*/
--
2.20.1
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