On Thu, 21 Mar 2019 at 13:43, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
>
> 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 72374e952e4b..36f557002005 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!
for us to do ?
Reviewed-by: Matthew Auld <matthew.william.auld@xxxxxxxxx>
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