Re: [PATCH v3] uapi/drm/i915: Document memory residency and Flat-CCS capability of obj

On 02/05/2022 17:15, Ramalingam C wrote:
> Capture the impact of memory region preference list of the objects, on
> their memory residency and Flat-CCS capability.
>
> v2:
>    Fix the Flat-CCS capability of an obj with {lmem, smem} preference
>    list [Thomas]
> v3:
>    Reworded the doc [Matt]
>
> Signed-off-by: Ramalingam C <[email protected]>
> cc: Matthew Auld <[email protected]>
> cc: Thomas Hellstrom <[email protected]>
> cc: Daniel Vetter <[email protected]>
> cc: Jon Bloomfield <[email protected]>
> cc: Lionel Landwerlin <[email protected]>
> cc: Kenneth Graunke <[email protected]>
> cc: [email protected]
> cc: Jordan Justen <[email protected]>
> cc: Tony Ye <[email protected]>
> Reviewed-by: Matthew Auld <[email protected]>
> ---
>   include/uapi/drm/i915_drm.h | 16 ++++++++++++++++
>   1 file changed, 16 insertions(+)
>
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index a2def7b27009..b7e1c2fe08dc 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -3443,6 +3443,22 @@ 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.
> + *
> + * 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 @regions 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 dosen't have the required information to
> + * decompress the userspace compressed objects.
> + *
> + * So I915 supports Flat-CCS, only on the objects which can reside only on
> + * I915_MEMORY_CLASS_DEVICE regions.


I think it's fine to assume Flat-CSS surface will always be in lmem.

I see no issue for the Anv Vulkan driver.


Maybe Nanley or Ken can speak for the Iris GL driver?


-Lionel


>    */
>   struct drm_i915_gem_create_ext_memory_regions {
>   	/** @base: Extension link. See struct i915_user_extension. */