Re: [PATCH 3/4] drm/i915/gt: Extend doc on Flat-CCS obj eviction

["Hellstrom, Thomas" <thomas.hellstrom@intel.com>]

On Thu, 2022-04-21 at 17:08 +0530, Ramalingam C wrote:
> Capture the eviction details for Flat-CCS capable lmem only objects
> and
> lmem objects with smem residency. This also captures the impact of
> eviction on  object's memory residency and Flat-CCS compression
> state.
> 
> Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_migrate.c | 36 ++++++++++++++++++-----
> --
>  1 file changed, 27 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_migrate.c
> b/drivers/gpu/drm/i915/gt/intel_migrate.c
> index 463a6a14b5f9..9d0d18950e76 100644
> --- a/drivers/gpu/drm/i915/gt/intel_migrate.c
> +++ b/drivers/gpu/drm/i915/gt/intel_migrate.c
> @@ -485,16 +485,34 @@ static bool wa_1209644611_applies(int ver, u32
> size)
>   * And CCS data can be copied in and out of CCS region through
>   * XY_CTRL_SURF_COPY_BLT. CPU can't access the CCS data directly.
>   *
> - * When we exhaust the lmem, if the object's placements support
> smem, then we can
> - * directly decompress the compressed lmem object into smem and
> start using it
> - * from smem itself.
> + * when we exhaust the lmem, we need to handle two types of flat-ccs
> capable
> + * objects for its eviction.
> + *   1) lmem only objects
> + *   2) lmem objects with smem residency option
>   *
> - * But when we need to swapout the compressed lmem object into a
> smem region
> - * though objects' placement doesn't support smem, then we copy the
> lmem content
> - * as it is into smem region along with ccs data (using
> XY_CTRL_SURF_COPY_BLT).
> - * When the object is referred, lmem content will be swaped in along
> with
> - * restoration of the CCS data (using XY_CTRL_SURF_COPY_BLT) at
> corresponding
> - * location.
> + * 1) lmem only objects:
> + *
> + * lmem backing memory can be temporarily evicted to smem, along
> with the
> + * auxiliary CCS state, where it can be potentially swapped-out at a
> later point,
> + * if required. If userspace later touches the evicted pages, then
> we always move
> + * the backing memory back to lmem, which includes restoring the
> saved CCS state,
> + * and potentially performing any required swap-in.
> + *
> + * In this scenario, objects' backing memory class and Flat-CCS
> state doesn't
> + * change.
> + *
> + * 2) lmem objects with smem residency option
> + *
> + * Lmem object with smem region in it's placement list, will be
> migrated into
> + * smem  by decompressing the content. I915 doesn't handle this kind
> of
> + * migration for Flat-CCS compressed objects yet.
> + *
> + * In this scenario, objects' backing memory class and Flat-CCS
> state changed,
> + * and userspace is not aware of it.
> + *
> + * In summary, when a userspace wants to be sure about the objects
> memory
> + * residency and flat-ccs compression state, then placement list
> can't have
> + * the lmem and smem together. Instead, object has to be lmem
> resident only.

For 2) I was under the impression that with flat CCS, these objects
need to be always uncompressed, since the kernel doesn't have the
needed information to decompress / compress. Or has this been changed
recently?

/Thomas




>   */
>  
>  static inline u32 *i915_flush_dw(u32 *cmd, u32 flags)