On Fri, 22 Aug 2025 10:29:10 -0400
Faith Ekstrand <fa...@gfxstrand.net> wrote:
> This enables syncing mapped GEM objects between the CPU and GPU via calls
> to dma_sync_*(). It's a bit annoying as it requires walking the sg_table
> so it's best if every driver doesn't hand-roll it.
>
> Signed-off-by: Faith Ekstrand <faith.ekstr...@collabora.com>
> ---
> drivers/gpu/drm/drm_gem_shmem_helper.c | 64 ++++++++++++++++++++++++++
> include/drm/drm_gem_shmem_helper.h | 3 ++
> 2 files changed, 67 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c
> b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 8ac0b1fa5287..50907c1fa11f 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -658,6 +658,70 @@ int drm_gem_shmem_mmap(struct drm_gem_shmem_object
> *shmem, struct vm_area_struct
> }
> EXPORT_SYMBOL_GPL(drm_gem_shmem_mmap);
>
> +/**
> + * drm_gem_shmem_sync_mmap - Sync memor-mapped data to/from the device
^ CPU-mapped
> + * @shmem: shmem GEM object
> + * @offset: Offset into the GEM object
> + * @size: Size of the area to sync
> + *
> + * Returns:
> + * 0 on success or a negative error code on failure.
> + */
> +int
> +drm_gem_shmem_sync_mmap(struct drm_gem_shmem_object *shmem,
> + size_t offset, size_t size,
> + enum dma_data_direction dir)
Feels weird to name this function sync_mmap when all it's being passed
is a region is a GEM, not a virtual address in the process address
space. I can also imagine this helper being used for kernel-mapped BOs,
making _mmap a bit of a lie. Maybe we should just go for _sync() or
_sync_cpu().
> +{
> + struct drm_device *dev = shmem->base.dev;
> + struct sg_table *sgt;
> + struct scatterlist *sgl;
> + unsigned int count;
> +
> + if (dir == DMA_NONE)
> + return 0;
> +
> + /* Don't bother if it's WC-mapped */
> + if (shmem->map_wc)
> + return 0;
> +
> + if (size == 0)
> + return 0;
> +
> + if (offset + size < offset || offset + size > shmem->base.size)
> + return -EINVAL;
> +
> + sgt = drm_gem_shmem_get_pages_sgt(shmem);
> + if (IS_ERR(sgt))
> + return PTR_ERR(sgt);
> +
> + for_each_sgtable_dma_sg(sgt, sgl, count) {
> + if (size == 0)
> + break;
> +
> + dma_addr_t paddr = sg_dma_address(sgl);
> + size_t len = sg_dma_len(sgl);
> +
> + if (len <= offset) {
> + offset -= len;
> + continue;
> + }
> +
> + paddr += offset;
> + len -= offset;
> + len = min_t(size_t, len, size);
> + size -= len;
> + offset = 0;
> +
> + if (dir == DMA_FROM_DEVICE)
> + dma_sync_single_for_cpu(dev->dev, paddr, len, dir);
> + else
> + dma_sync_single_for_device(dev->dev, paddr, len, dir);
> + }
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(drm_gem_shmem_sync_mmap);
> +
> /**
> * drm_gem_shmem_print_info() - Print &drm_gem_shmem_object info for debugfs
> * @shmem: shmem GEM object
> diff --git a/include/drm/drm_gem_shmem_helper.h
> b/include/drm/drm_gem_shmem_helper.h
> index 92f5db84b9c2..8e057a8e6c83 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -121,6 +121,9 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object
> *shmem,
> void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
> struct iosys_map *map);
> int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct
> vm_area_struct *vma);
> +int drm_gem_shmem_sync_mmap(struct drm_gem_shmem_object *shmem,
> + size_t offset, size_t size,
> + enum dma_data_direction dir);
>
> int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem);
> void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem);
