On Thu, Mar 21, 2019 at 04:19:08PM +0000, Chris Wilson 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;
Hopefully no one is relying on read_domains==0 meaning cpu domain.
That seems to be how this was handled before.
Or maybe we want -EIVNAL here?
> +
> 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;
> + }
Hard to argue with that logic.
Haven't paid too much attention to this area lately but this
makes sense to me.
Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> +
> + /*
> + * 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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--
Ville Syrjälä
Intel
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