[PATCH v2 4/4] uapi/drm/i915: Document memory residency and Flat-CCS capability of obj

[Matthew Auld]

On 25/04/2022 17:24, Ramalingam C wrote:
> Capture the impact of memory region preference list of an object, on
> their memory residency and Flat-CCS capability of the objects.
> 
> v2:
>    Fix the Flat-CCS capability of an obj with {lmem, smem} preference
>    list [Thomas]
> 
> Signed-off-by: Ramalingam C <ramalingam.c at intel.com>
> cc: Matthew Auld <matthew.auld at intel.com>
> cc: Thomas Hellstrom <thomas.hellstrom at linux.intel.com>
> ---
>   include/uapi/drm/i915_drm.h | 18 ++++++++++++++++++
>   1 file changed, 18 insertions(+)
> 
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 35ca528803fd..ad191ed6547c 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -3393,6 +3393,24 @@ struct drm_i915_gem_create_ext {
>    * At which point we get the object handle in &drm_i915_gem_create_ext.handle,
>    * along with the final object size in &drm_i915_gem_create_ext.size, which
>    * should account for any rounding up, if required.
> + *
> + * Objects with multiple memory regions in the preference list will be backed
> + * by one of the memory regions mentioned in the preference list. Though I915
> + * tries to honour the order of the memory regions in the preference list,
> + * based on the memory pressure of the regions, objects' backing region
> + * will be selected.
> + *
> + * Userspace has no means of knowing the backing region for such objects.

"Note that userspace has no means of knowing the current backing region 
for objects where @num_regions is larger than one. The kernel will only 
ensure that the priority order of the @regions array is honoured, either 
when initially placing the object, or when moving memory around due to 
memory pressure."

> + *
> + * On Flat-CCS capable HW, compression is supported for the objects residing
> + * in I915_MEMORY_CLASS_DEVICE. When such objects (compressed) has other
> + * memory class in preference list and migrated (by I915, due to memory
> + * constrain) to the non I915_MEMORY_CLASS_DEVICE region, then I915 needs to
> + * decompress the content. But I915 dont have the required information to

"doesn't", also prefer @regions etc instead of "preference list"

Anyway,
Reviewed-by: Matthew Auld <matthew.auld at intel.com>

> + * decompress the userspace compressed objects.
> + *
> + * So I915 supports Flat-CCS, only on the objects which can reside only on
> + * I915_MEMORY_CLASS_DEVICE regions. >    */
>   struct drm_i915_gem_create_ext_memory_regions {
>   	/** @base: Extension link. See struct i915_user_extension. */