Re: [PATCH v2 3/4] drm/i915/gt: Document the eviction of the Flat-CCS objects

[Matthew Auld <matthew.auld@intel.com>]

On 25/04/2022 17:24, Ramalingam C wrote:
> Capture the eviction details for Flat-CCS capable, lmem objects.
> 
> v2:
>    Fix the Flat-ccs capbility of lmem obj with smem residency
>    possibility [Thomas]
> 
> Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
> cc: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
> cc: Matthew Auld <matthew.auld@intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_migrate.c | 23 ++++++++++++++---------
>   1 file changed, 14 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..930e0fd9795f 100644
> --- a/drivers/gpu/drm/i915/gt/intel_migrate.c
> +++ b/drivers/gpu/drm/i915/gt/intel_migrate.c
> @@ -485,16 +485,21 @@ 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.
> + * I915 supports Flat-CCS on lmem only objects. When an objects has the smem in

"When an object has smem in"

> + * its preference list, on memory pressure, i915 needs to migarte the lmem

"migrate"

> + * content into smem. If the lmem object is Flat-CCS compressed by userspace,
> + * then i915 needs to decompress it. But I915 lack the required information
> + * for such decompression. Hence I915 supports Flat-CCS only on lmem only objects.
>    *
> - * 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.
> + * when we exhaust the lmem, Flat-CCS capable objects' lmem backing memory can

"When"

Otherwise,
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

> + * 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.
> + *
> + * For the migration of the lmem objects with smem in placement list, such as
> + * {lmem, smem}, objects are treated as non Flat-CCS capable objects.
>    */
>   
>   static inline u32 *i915_flush_dw(u32 *cmd, u32 flags)