On Mon, 3 Nov 2025 16:43:12 +0000 Akash Goel <[email protected]> wrote:
> On 10/30/25 14:05, Boris Brezillon wrote: > > From: Loïc Molinari <[email protected]> > > > > Will be used by the UMD to optimize CPU accesses to buffers > > that are frequently read by the CPU, or on which the access > > pattern makes non-cacheable mappings inefficient. > > > > Mapping buffers CPU-cached implies taking care of the CPU > > cache maintenance in the UMD, unless the GPU is IO coherent. > > > > v2: > > - Add more to the commit message > > - Tweak the doc > > - Make sure we sync the section of the BO pointing to the CS > > syncobj before we read its seqno > > > > v3: > > - Fix formatting/spelling issues > > > > v4: > > - Add Steve's R-b > > > > v5: > > - Drop Steve's R-b (changes in the ioctl semantics requiring > > new review) > > > > Signed-off-by: Loïc Molinari <[email protected]> > > Signed-off-by: Boris Brezillon <[email protected]> > > --- > > drivers/gpu/drm/panthor/panthor_drv.c | 7 ++++- > > drivers/gpu/drm/panthor/panthor_gem.c | 37 +++++++++++++++++++++++-- > > drivers/gpu/drm/panthor/panthor_sched.c | 18 ++++++++++-- > > include/uapi/drm/panthor_drm.h | 12 ++++++++ > > 4 files changed, 69 insertions(+), 5 deletions(-) > > > > diff --git a/drivers/gpu/drm/panthor/panthor_drv.c > > b/drivers/gpu/drm/panthor/panthor_drv.c > > index c07fc5dcd4a1..4e915f5ef3fa 100644 > > --- a/drivers/gpu/drm/panthor/panthor_drv.c > > +++ b/drivers/gpu/drm/panthor/panthor_drv.c > > @@ -900,7 +900,8 @@ static int panthor_ioctl_vm_destroy(struct drm_device > > *ddev, void *data, > > return panthor_vm_pool_destroy_vm(pfile->vms, args->id); > > } > > > > -#define PANTHOR_BO_FLAGS DRM_PANTHOR_BO_NO_MMAP > > +#define PANTHOR_BO_FLAGS (DRM_PANTHOR_BO_NO_MMAP | \ > > + DRM_PANTHOR_BO_WB_MMAP) > > > > static int panthor_ioctl_bo_create(struct drm_device *ddev, void *data, > > struct drm_file *file) > > @@ -919,6 +920,10 @@ static int panthor_ioctl_bo_create(struct drm_device > > *ddev, void *data, > > goto out_dev_exit; > > } > > > > + if ((args->flags & DRM_PANTHOR_BO_NO_MMAP) && > > + (args->flags & DRM_PANTHOR_BO_WB_MMAP)) > > + return -EINVAL; > > + > > if (args->exclusive_vm_id) { > > vm = panthor_vm_pool_get_vm(pfile->vms, > > args->exclusive_vm_id); > > if (!vm) { > > diff --git a/drivers/gpu/drm/panthor/panthor_gem.c > > b/drivers/gpu/drm/panthor/panthor_gem.c > > index 1b1e98c61b8c..479a779ee59d 100644 > > --- a/drivers/gpu/drm/panthor/panthor_gem.c > > +++ b/drivers/gpu/drm/panthor/panthor_gem.c > > @@ -58,6 +58,39 @@ static void panthor_gem_debugfs_set_usage_flags(struct > > panthor_gem_object *bo, u > > static void panthor_gem_debugfs_bo_init(struct panthor_gem_object *bo) {} > > #endif > > > > +static bool > > +should_map_wc(struct panthor_gem_object *bo, struct panthor_vm > > *exclusive_vm) > > +{ > > + struct panthor_device *ptdev = container_of(bo->base.base.dev, struct > > panthor_device, base); > > + > > + /* We can't do uncached mappings if the device is coherent, > > + * because the zeroing done by the shmem layer at page allocation > > + * time happens on a cached mapping which isn't CPU-flushed (at least > > + * not on Arm64 where the flush is deferred to PTE setup time, and > > + * only done conditionally based on the mapping permissions). We can't > > + * rely on dma_map_sgtable()/dma_sync_sgtable_for_xxx() either to > > flush > > + * those, because they are NOPed if dma_dev_coherent() returns true. > > + * > > + * FIXME: Note that this problem is going to pop up again when we > > + * decide to support mapping buffers with the NO_MMAP flag as > > + * non-shareable (AKA buffers accessed only by the GPU), because we > > + * need the same CPU flush to happen after page allocation, otherwise > > + * there's a risk of data leak or late corruption caused by a dirty > > + * cacheline being evicted. At this point we'll need a way to force > > + * CPU cache maintenance regardless of whether the device is coherent > > + * or not. > > + */ > > + if (ptdev->coherent) > > + return false; > > + > > + /* Cached mappings are explicitly requested, so no write-combine. */ > > + if (bo->flags & DRM_PANTHOR_BO_WB_MMAP) > > + return false; > > + > > + /* The default is write-combine. */ > > + return true; > > +} > > + > > static void panthor_gem_free_object(struct drm_gem_object *obj) > > { > > struct panthor_gem_object *bo = to_panthor_bo(obj); > > @@ -152,6 +185,7 @@ panthor_kernel_bo_create(struct panthor_device *ptdev, > > struct panthor_vm *vm, > > bo = to_panthor_bo(&obj->base); > > kbo->obj = &obj->base; > > bo->flags = bo_flags; > > + bo->base.map_wc = should_map_wc(bo, vm); > > > > if (vm == panthor_fw_vm(ptdev)) > > debug_flags |= PANTHOR_DEBUGFS_GEM_USAGE_FLAG_FW_MAPPED; > > @@ -255,7 +289,6 @@ static const struct drm_gem_object_funcs > > panthor_gem_funcs = { > > */ > > struct drm_gem_object *panthor_gem_create_object(struct drm_device *ddev, > > size_t size) > > { > > - struct panthor_device *ptdev = container_of(ddev, struct > > panthor_device, base); > > struct panthor_gem_object *obj; > > > > obj = kzalloc(sizeof(*obj), GFP_KERNEL); > > @@ -263,7 +296,6 @@ struct drm_gem_object *panthor_gem_create_object(struct > > drm_device *ddev, size_t > > return ERR_PTR(-ENOMEM); > > > > obj->base.base.funcs = &panthor_gem_funcs; > > - obj->base.map_wc = !ptdev->coherent; > > mutex_init(&obj->label.lock); > > > > panthor_gem_debugfs_bo_init(obj); > > @@ -298,6 +330,7 @@ panthor_gem_create_with_handle(struct drm_file *file, > > > > bo = to_panthor_bo(&shmem->base); > > bo->flags = flags; > > + bo->base.map_wc = should_map_wc(bo, exclusive_vm); > > > > if (exclusive_vm) { > > bo->exclusive_vm_root_gem = panthor_vm_root_gem(exclusive_vm); > > diff --git a/drivers/gpu/drm/panthor/panthor_sched.c > > b/drivers/gpu/drm/panthor/panthor_sched.c > > index f5e01cb16cfc..7d5da5717de2 100644 > > --- a/drivers/gpu/drm/panthor/panthor_sched.c > > +++ b/drivers/gpu/drm/panthor/panthor_sched.c > > @@ -868,8 +868,11 @@ panthor_queue_get_syncwait_obj(struct panthor_group > > *group, struct panthor_queue > > struct iosys_map map; > > int ret; > > > > - if (queue->syncwait.kmap) > > - return queue->syncwait.kmap + queue->syncwait.offset; > > + if (queue->syncwait.kmap) { > > + bo = container_of(queue->syncwait.obj, > > + struct panthor_gem_object, base.base); > > + goto out_sync; > > + } > > > > bo = panthor_vm_get_bo_for_va(group->vm, > > queue->syncwait.gpu_va, > > @@ -886,6 +889,17 @@ panthor_queue_get_syncwait_obj(struct panthor_group > > *group, struct panthor_queue > > if (drm_WARN_ON(&ptdev->base, !queue->syncwait.kmap)) > > goto err_put_syncwait_obj; > > > > +out_sync: > > + /* Make sure the CPU caches are invalidated before the seqno is read. > > + * drm_gem_shmem_sync() is a NOP if map_wc=false, so no need to check > > > > Sorry nitpick. > > IIUC, drm_gem_shmem_sync() would be a NOP if 'map_wc' is true. Oops, will fix that. > > > > > + * it here. > > + */ > > + drm_gem_shmem_sync(&bo->base, queue->syncwait.offset, > > + queue->syncwait.sync64 ? > > + sizeof(struct panthor_syncobj_64b) : > > + sizeof(struct panthor_syncobj_32b), > > + DRM_GEM_SHMEM_SYNC_CPU_CACHE_FLUSH_AND_INVALIDATE); > > + > > return queue->syncwait.kmap + queue->syncwait.offset; > > > > err_put_syncwait_obj: > > diff --git a/include/uapi/drm/panthor_drm.h b/include/uapi/drm/panthor_drm.h > > index 7eec9f922183..57e2f5ffa03c 100644 > > --- a/include/uapi/drm/panthor_drm.h > > +++ b/include/uapi/drm/panthor_drm.h > > @@ -681,6 +681,18 @@ struct drm_panthor_vm_get_state { > > enum drm_panthor_bo_flags { > > /** @DRM_PANTHOR_BO_NO_MMAP: The buffer object will never be > > CPU-mapped in userspace. */ > > DRM_PANTHOR_BO_NO_MMAP = (1 << 0), > > + > > + /** > > + * @DRM_PANTHOR_BO_WB_MMAP: Force "Write-Back Cacheable" CPU mapping. > > + * > > + * CPU map the buffer object in userspace by forcing the "Write-Back > > + * Cacheable" cacheability attribute. The mapping otherwise uses the > > + * "Non-Cacheable" attribute if the GPU is not IO coherent. > > + * > > + * Can't be set if exclusive_vm_id=0 (only private BOs can be mapped > > + * cacheable). > > Sorry Boris, I may have misinterpreted the code. > > As per the comment, DRM_PANTHOR_BO_WB_MMAP flag should be rejected if > 'exclusive_vm' is NULL. But I don't see any check for 'exclusive_vm' > pointer inside should_map_wc(). You're right, I had this behavior enforced at some point, and dropped it after adding {begin,end}_cpu_access() implementations to panthor. I'll revisit the comment or re-introduce the check in v6 based on how the review process goes.
