[PATCH] drm/xe/bo: optimise CCS case for WB pages

[Souza, Jose]

On Fri, 2025-05-16 at 16:38 +0100, Matthew Auld wrote:
> Dealing with CCS state is significant on LNL+, where we end up clearing
> the compression state on every page alloc using the blitter for user
> buffers, including also saving and restoring it when moving between
> domains, plus we need to alloc extra pages to hold the raw CCS state for
> the save step.
> 
> However all compression PAT modes, on platforms like LNL, also require
> coh_none, meaning that only WC memory can use compression in the first
> place. With this we can be sneaky and completely ignore CCS for WB
> buffers, which is likely the common case anyway. This would then skip
> all blitter moves/clears between sys <-> tt and then also means we can
> drop the extra CCS pages.
> 
> This should be safe since there is no way to interact with the
> compression state (potentially uncleared) without using a PAT enabled
> index (which is rejected at bind), including if trying to be malicious
> and copy the raw CCS state from userpace, which should give back all
> zeroes if the src surface (indirect) is lacking compressed PAT index.

At least on Mesa case we know at gem_create time if bo is going to have a compressed or not PAT index.
We could optimize it even further by having a flag in drm_xe_gem_create, something on flags or cpu_caching...

Anyways this LGTM:

Reviewed-by: José Roberto de Souza <jose.souza at intel.com>

> 
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> Cc: Satyanarayana K V P <satyanarayana.k.v.p at intel.com>
> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Cc: Matthew Brost <matthew.brost at intel.com>
> ---
>  drivers/gpu/drm/xe/xe_bo.c  | 8 ++++++++
>  drivers/gpu/drm/xe/xe_pat.c | 3 ++-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index d99d91fe8aa9..3fafdcb8d95b 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -2982,6 +2982,14 @@ bool xe_bo_needs_ccs_pages(struct xe_bo *bo)
>  	if (IS_DGFX(xe) && (bo->flags & XE_BO_FLAG_SYSTEM))
>  		return false;
>  
> +	/*
> +	 * Compression implies coh_none, therefore we know for sure that WB
> +	 * memory can't currently use compression, which is likely one of the
> +	 * common cases.
> +	 */
> +	if (bo->cpu_caching == DRM_XE_GEM_CPU_CACHING_WB)
> +		return false;
> +
>  	return true;
>  }
>  
> diff --git a/drivers/gpu/drm/xe/xe_pat.c b/drivers/gpu/drm/xe/xe_pat.c
> index 30fdbdb9341e..38a6a49c1b2a 100644
> --- a/drivers/gpu/drm/xe/xe_pat.c
> +++ b/drivers/gpu/drm/xe/xe_pat.c
> @@ -103,7 +103,8 @@ static const struct xe_pat_table_entry xelpg_pat_table[] = {
>   *
>   * Note: There is an implicit assumption in the driver that compression and
>   * coh_1way+ are mutually exclusive. If this is ever not true then userptr
> - * and imported dma-buf from external device will have uncleared ccs state.
> + * and imported dma-buf from external device will have uncleared ccs state. See
> + * also xe_bo_needs_ccs_pages().
>   */
>  #define XE2_PAT(no_promote, comp_en, l3clos, l3_policy, l4_policy, __coh_mode) \
>  	{ \