[Intel-gfx] [PATCH 05/11] drm/i915: Support for creating Stolen memory backed objects

Chris Wilson chris at chris-wilson.co.uk
Thu Jan 14 02:24:08 PST 2016

On Thu, Jan 14, 2016 at 11:46:41AM +0530, ankitprasad.r.sharma at intel.com wrote:
> From: Ankitprasad Sharma <ankitprasad.r.sharma at intel.com>
> Extend the drm_i915_gem_create structure to add support for
> creating Stolen memory backed objects. Added a new flag through
> which user can specify the preference to allocate the object from
> stolen memory, which if set, an attempt will be made to allocate
> the object from stolen memory subject to the availability of
> free space in the stolen region.
> v2: Rebased to the latest drm-intel-nightly (Ankit)
> v3: Changed versioning of GEM_CREATE param, added new comments (Tvrtko)
> v4: Changed size from 32b to 64b to prevent userspace overflow (Tvrtko)
> Corrected function arguments ordering (Chris)
> v5: Corrected function name (Chris)
> v6: Updated datatype for flags to keep sizeof(drm_i915_gem_create) u64
> aligned (Chris)
> v7: Use first 8 bits of gem_create flags for placement (Chris), Add helper
> function for object allocation from stolen region (Ankit)
> Testcase: igt/gem_stolen
> Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma at intel.com>

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

> +	/**
> +	 * Requested flags (currently used for placement
> +	 * (which memory domain))
> +	 *
> +	 * You can request that the object be created from special memory
> +	 * rather than regular system pages using this parameter. Such
> +	 * irregular objects may have certain restrictions (such as CPU
> +	 * access to a stolen object is verboten).
> +	 *
> +	 * This can be used in the future for other purposes too
> +	 * e.g. specifying tiling/caching/madvise
> +	 */
> +	__u64 flags;
> +#define I915_CREATE_PLACEMENT_NORMAL 	0 /* standard swappable bo  */
> +#define I915_CREATE_PLACEMENT_STOLEN 	1 /* Cannot use CPU mmaps */

/* Allocate the object from memory reserved for the igfx (stolen).
 * Objects allocated from stolen are restricted in the API they can use,
 * as direct CPU access to stolen memory is prohibited by the system.
 * This means that you cannot use a regular CPU mmap (either using WB
 * or with the WC extension). You can still use a GTT mmap, pwrite,
 * pread and pass it around for use by execbuffer and between processes
 * like normal.
 * Stolen memory is a very limited resource and certain functions of the
 * hardware can only work from within stolen memory. Userspace's
 * allocations may be evicted from stolen and moved to normal memory as
 * required. If the allocation is marked as purgeable (using madvise),
 * the allocation will be dropped and further access to the object's
 * backing storage will result in -EFAULT. Stolen objects will also be
 * migrated to normal memory across suspend and resume, as the stolen
 * memory is not preserved.
 * Stolen memory is regarded as a resource placement hint, most suitable
 * for medium-sized buffers that are only accessed by the GPU and can be
 * discarded.

Chris Wilson, Intel Open Source Technology Centre

More information about the Intel-gfx mailing list