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.
Signed-off-by: Thomas Hellström thomas.hellstrom@linux.intel.com Fixes: 622f709ca629 ("drm/xe/uapi: Add support for CPU caching mode") Cc: Pallavi Mishra pallavi.mishra@intel.com Cc: Matthew Auld matthew.auld@intel.com Cc: dri-devel@lists.freedesktop.org Cc: Joonas Lahtinen joonas.lahtinen@linux.intel.com Cc: Effie Yu effie.yu@intel.com Cc: Matthew Brost matthew.brost@intel.com Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com Cc: Jose Souza jose.souza@intel.com Cc: Michal Mrozek michal.mrozek@intel.com Cc: stable@vger.kernel.org # 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.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
Was this supposed to say "including evicted vram?"
Matt
* 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.-- 2.44.0