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Commit 754a2544 authored by Chris Wilson's avatar Chris Wilson
Browse files

drm/i915: Skip object locking around a no-op set-domain ioctl 



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: 40e62d5d ("drm/i915: Acquire the backing storage outside of struct_mutex in set-domain")
Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: default avatarMatthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321161908.8007-2-chris@chris-wilson.co.uk
parent a679f58d

drivers/gpu/drm/i915/i915_gem.c

+23 −3
Original line number Diff line number Diff line
@@ -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 write domain? Nothing for us to do!

	 *

	 * 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.

	 */