[Intel-gfx] [PATCH v2 13/15] drm/i915/uapi: add NEEDS_CPU_ACCESS hint

[Thomas Hellström]

On 2/10/22 13:13, Matthew Auld wrote:
> If set, force the allocation to be placed in the mappable portion of
> LMEM. One big restriction here is that system memory must be given as a
> potential placement for the object, that way we can always spill the
> object into system memory if we can't make space.
>
> XXX: Still very much WIP and needs IGTs. Including now just for the sake
> of having more complete picture.
>
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> ---
>   drivers/gpu/drm/i915/gem/i915_gem_create.c | 28 ++++++++++++-------
>   include/uapi/drm/i915_drm.h                | 31 +++++++++++++++++++++-
>   2 files changed, 49 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_create.c b/drivers/gpu/drm/i915/gem/i915_gem_create.c
> index cc9ddb943f96..2737977f81a4 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_create.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_create.c
> @@ -238,6 +238,7 @@ struct create_ext {
>   	struct drm_i915_private *i915;
>   	struct intel_memory_region *placements[INTEL_REGION_UNKNOWN];
>   	unsigned int n_placements;
> +	unsigned int placement_mask;
>   	unsigned long flags;
>   };
>   
> @@ -334,6 +335,7 @@ static int set_placements(struct drm_i915_gem_create_ext_memory_regions *args,
>   	for (i = 0; i < args->num_regions; i++)
>   		ext_data->placements[i] = placements[i];
>   
> +	ext_data->placement_mask = mask;
>   	return 0;
>   
>   out_dump:
> @@ -408,7 +410,7 @@ i915_gem_create_ext_ioctl(struct drm_device *dev, void *data,
>   	struct drm_i915_gem_object *obj;
>   	int ret;
>   
> -	if (args->flags)
> +	if (args->flags & ~I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS)
>   		return -EINVAL;
>   
>   	ret = i915_user_extensions(u64_to_user_ptr(args->extensions),
> @@ -424,14 +426,22 @@ i915_gem_create_ext_ioctl(struct drm_device *dev, void *data,
>   		ext_data.n_placements = 1;
>   	}
>   
> -	/*
> -	 * TODO: add a userspace hint to force CPU_ACCESS for the object, which
> -	 * can override this.
> -	 */
> -	if (!IS_DG1(i915) && (ext_data.n_placements > 1 ||
> -			      ext_data.placements[0]->type !=
> -			      INTEL_MEMORY_SYSTEM))
> -		ext_data.flags |= I915_BO_ALLOC_GPU_ONLY;
> +	if (args->flags & I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS) {
> +		if (ext_data.n_placements == 1)
> +			return -EINVAL;
> +
> +		/*
> +		 * We always need to be able to spill to system memory, if we
> +		 * can't place in the mappable part of LMEM.
> +		 */
> +		if (!(ext_data.placement_mask & BIT(INTEL_REGION_SMEM)))
> +			return -EINVAL;
> +	} else {
> +		if (!IS_DG1(i915) && (ext_data.n_placements > 1 ||
> +				      ext_data.placements[0]->type !=
> +				      INTEL_MEMORY_SYSTEM))
> +			ext_data.flags |= I915_BO_ALLOC_GPU_ONLY;
> +	}
>   
>   	obj = __i915_gem_object_create_user_ext(i915, args->size,
>   						ext_data.placements,
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 914ebd9290e5..925685bd261e 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -3157,7 +3157,36 @@ struct drm_i915_gem_create_ext {
>   	 * Object handles are nonzero.
>   	 */
>   	__u32 handle;
> -	/** @flags: MBZ */
> +	/**
> +	 * @flags: Optional flags.
> +	 *
> +	 * Supported values:
> +	 *
> +	 * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS - Signal to the kernel that
> +	 * the object will need to be accessed via the CPU.
> +	 *
> +	 * Only valid when placing objects in I915_MEMORY_CLASS_DEVICE, and
> +	 * only strictly required on platforms where only some of the device
> +	 * memory is directly visible or mappable through the CPU, like on DG2+.
> +	 *
> +	 * One of the placements MUST also be I915_MEMORY_CLASS_SYSTEM, to
> +	 * ensure we can always spill the allocation to system memory, if we
> +	 * can't place the object in the mappable part of
> +	 * I915_MEMORY_CLASS_DEVICE.
> +	 *
> +	 * Note that buffers that need to be captured with EXEC_OBJECT_CAPTURE,
> +	 * will need to enable this hint, if the object can also be placed in
> +	 * I915_MEMORY_CLASS_DEVICE, starting from DG2+. The execbuf call will
> +	 * throw an error otherwise. This also means that such objects will need
> +	 * I915_MEMORY_CLASS_SYSTEM set as a possible placement.

It looks like Jon still wants the capture part of the uAPI to remain 
like this. In that case, this patch looks good to me. Since you mention 
it's Wip, I figure we need an ack from arch team?

/Thomas




> +	 *
> +	 * Without this hint, the kernel will assume that non-mappable
> +	 * I915_MEMORY_CLASS_DEVICE is preferred for this object. Note that the
> +	 * kernel can still migrate the object to the mappable part, as a last
> +	 * resort, if userspace ever CPU faults this object, but this might be
> +	 * expensive, and so ideally should be avoided.
> +	 */
> +#define I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS (1 << 0)
>   	__u32 flags;
>   	/**
>   	 * @extensions: The chain of extensions to apply to this object.