On Wed, Jun 19, 2024 at 06:39:04PM +0200, Thomas Hellström wrote:
> The caching mode for buffer objects with VRAM as a possible
> placement was forced to write-combined, regardless of placement.
>
> However, write-combined system memory is expensive to allocate and
> even though it is pooled, the pool is expensive to shrink, since
> it involves global CPU TLB flushes.
>
> Moreover write-combined system memory from TTM is only reliably
> available on x86 and DGFX doesn't have an x86 restriction.
>
> So regardless of the cpu caching mode selected for a bo,
> internally use write-back caching mode for system memory on DGFX.
>
> Coherency is maintained, but user-space clients may perceive a
> difference in cpu access speeds.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> Fixes: 622f709ca629 ("drm/xe/uapi: Add support for CPU caching mode")
> Cc: Pallavi Mishra <pallavi.mishra@xxxxxxxxx>
> Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Effie Yu <effie.yu@xxxxxxxxx>
> Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Jose Souza <jose.souza@xxxxxxxxx>
> Cc: Michal Mrozek <michal.mrozek@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v6.8+
> ---
> drivers/gpu/drm/xe/xe_bo.c | 47 +++++++++++++++++++-------------
> drivers/gpu/drm/xe/xe_bo_types.h | 3 +-
> include/uapi/drm/xe_drm.h | 8 +++++-
> 3 files changed, 37 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index 65c696966e96..31192d983d9e 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -343,7 +343,7 @@ static struct ttm_tt *xe_ttm_tt_create(struct ttm_buffer_object *ttm_bo,
> struct xe_device *xe = xe_bo_device(bo);
> struct xe_ttm_tt *tt;
> unsigned long extra_pages;
> - enum ttm_caching caching;
> + enum ttm_caching caching = ttm_cached;
> int err;
>
> tt = kzalloc(sizeof(*tt), GFP_KERNEL);
> @@ -357,26 +357,35 @@ static struct ttm_tt *xe_ttm_tt_create(struct ttm_buffer_object *ttm_bo,
> extra_pages = DIV_ROUND_UP(xe_device_ccs_bytes(xe, bo->size),
> PAGE_SIZE);
>
> - switch (bo->cpu_caching) {
> - case DRM_XE_GEM_CPU_CACHING_WC:
> - caching = ttm_write_combined;
> - break;
> - default:
> - caching = ttm_cached;
> - break;
> - }
> -
> - WARN_ON((bo->flags & XE_BO_FLAG_USER) && !bo->cpu_caching);
> -
> /*
> - * Display scanout is always non-coherent with the CPU cache.
> - *
> - * For Xe_LPG and beyond, PPGTT PTE lookups are also non-coherent and
> - * require a CPU:WC mapping.
> + * DGFX system memory is always WB / ttm_cached, since
> + * other caching modes are only supported on x86. DGFX
> + * GPU system memory accesses are always coherent with the
> + * CPU.
> */
> - if ((!bo->cpu_caching && bo->flags & XE_BO_FLAG_SCANOUT) ||
> - (xe->info.graphics_verx100 >= 1270 && bo->flags & XE_BO_FLAG_PAGETABLE))
> - caching = ttm_write_combined;
> + if (!IS_DGFX(xe)) {
> + switch (bo->cpu_caching) {
> + case DRM_XE_GEM_CPU_CACHING_WC:
> + caching = ttm_write_combined;
> + break;
> + default:
> + caching = ttm_cached;
> + break;
> + }
> +
> + WARN_ON((bo->flags & XE_BO_FLAG_USER) && !bo->cpu_caching);
> +
> + /*
> + * Display scanout is always non-coherent with the CPU cache.
> + *
> + * For Xe_LPG and beyond, PPGTT PTE lookups are also
> + * non-coherent and require a CPU:WC mapping.
> + */
> + if ((!bo->cpu_caching && bo->flags & XE_BO_FLAG_SCANOUT) ||
> + (xe->info.graphics_verx100 >= 1270 &&
> + bo->flags & XE_BO_FLAG_PAGETABLE))
> + caching = ttm_write_combined;
> + }
>
> if (bo->flags & XE_BO_FLAG_NEEDS_UC) {
> /*
> diff --git a/drivers/gpu/drm/xe/xe_bo_types.h b/drivers/gpu/drm/xe/xe_bo_types.h
> index 86422e113d39..10450f1fbbde 100644
> --- a/drivers/gpu/drm/xe/xe_bo_types.h
> +++ b/drivers/gpu/drm/xe/xe_bo_types.h
> @@ -66,7 +66,8 @@ struct xe_bo {
>
> /**
> * @cpu_caching: CPU caching mode. Currently only used for userspace
> - * objects.
> + * objects. Exceptions are system memory on DGFX, which is always
> + * WB.
> */
> u16 cpu_caching;
>
> diff --git a/include/uapi/drm/xe_drm.h b/include/uapi/drm/xe_drm.h
> index 93e00be44b2d..1189b3044723 100644
> --- a/include/uapi/drm/xe_drm.h
> +++ b/include/uapi/drm/xe_drm.h
> @@ -783,7 +783,13 @@ struct drm_xe_gem_create {
> #define DRM_XE_GEM_CPU_CACHING_WC 2
> /**
> * @cpu_caching: The CPU caching mode to select for this object. If
> - * mmaping the object the mode selected here will also be used.
> + * mmaping the object the mode selected here will also be used. The
> + * exception is when mapping system memory (including evicted
> + * system memory) on discrete GPUs. The caching mode selected will
> + * then be overridden to DRM_XE_GEM_CPU_CACHING_WB, and coherency
> + * between GPU- and CPU is guaranteed. The caching mode of
> + * existing CPU-mappings will be updated transparently to
> + * user-space clients.
> */
> __u16 cpu_caching;
> /** @pad: MBZ */
> --
> 2.44.0
>
