Re: [PATCH 2/8] dma-heap: Provide accessors so that in-kernel drivers can allocate dmabufs from specific heaps

Boris Brezillon
2026-05-05T15:45:29Z

On Tue, 5 May 2026 16:05:08 +0200
Ketil Johnsen <ketil.johnsen@arm.com> wrote:

> From: John Stultz <jstultz@google.com>
>
> This allows drivers who don't want to create their own
> DMA-BUF exporter to be able to allocate DMA-BUFs directly
> from existing DMA-BUF Heaps.
>
> There is some concern that the premise of DMA-BUF heaps is
> that userland knows better about what type of heap memory
> is needed for a pipeline, so it would likely be best for
> drivers to import and fill DMA-BUFs allocated by userland
> instead of allocating one themselves, but this is still
> up for debate.

I think this commit message needs to be updated with more details
around what it's actually needed for here (driver needing protected
buffer to boot FW and expose a char device, and no clean way to pass
dmabufs around before this cdev is exposed).

>
> Signed-off-by: John Stultz <jstultz@google.com>
> Signed-off-by: T.J. Mercier <tjmercier@google.com>
> Signed-off-by: Yong Wu <yong.wu@mediatek.com>
> [Yong: Fix the checkpatch alignment warning]
> Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
> Signed-off-by: Florent Tomasin <florent.tomasin@arm.com>
> [Florent: Rebase]
> Signed-off-by: Ketil Johnsen <ketil.johnsen@arm.com>
> [Ketil: Rebase]
> ---
> drivers/dma-buf/dma-heap.c | 80 ++++++++++++++++++++++++++++++--------
> include/linux/dma-heap.h | 6 +++
> 2 files changed, 70 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c
> index 9fd365ddbd517..854d40d789ff2 100644
> --- a/drivers/dma-buf/dma-heap.c
> +++ b/drivers/dma-buf/dma-heap.c
> @@ -57,12 +57,24 @@ module_param(mem_accounting, bool, 0444);
> MODULE_PARM_DESC(mem_accounting,
> "Enable cgroup-based memory accounting for dma-buf heap allocations (default=false).");
>
> -static int dma_heap_buffer_alloc(struct dma_heap *heap, size_t len,
> - u32 fd_flags,
> - u64 heap_flags)
> +/**
> + * dma_heap_buffer_alloc - Allocate dma-buf from a dma_heap
> + * @heap: DMA-Heap to allocate from
> + * @len: size to allocate in bytes
> + * @fd_flags: flags to set on returned dma-buf fd
> + * @heap_flags: flags to pass to the dma heap
> + *
> + * This is for internal dma-buf allocations only. Free returned buffers with dma_buf_put().
> + */
> +struct dma_buf *dma_heap_buffer_alloc(struct dma_heap *heap, size_t len,
> + u32 fd_flags,
> + u64 heap_flags)
> {
> - struct dma_buf *dmabuf;
> - int fd;
> + if (fd_flags & ~DMA_HEAP_VALID_FD_FLAGS)
> + return ERR_PTR(-EINVAL);
> +
> + if (heap_flags & ~DMA_HEAP_VALID_HEAP_FLAGS)
> + return ERR_PTR(-EINVAL);

I'd probably move the flags checks to dma_heap_buffer_alloc() in a
separate patch, to keep the diff easier to read. Same for the
dma_heap_buffer_alloc()/dma_heap_bufferfd_alloc() split, though I'm not
too sure we need dma_heap_bufferfd_alloc(), we could just move the FD
allocation directly in dma_heap_ioctl_allocate().

>
> /*
> * Allocations from all heaps have to begin
> @@ -70,9 +82,20 @@ static int dma_heap_buffer_alloc(struct dma_heap *heap, size_t len,
> */
> len = PAGE_ALIGN(len);
> if (!len)
> - return -EINVAL;
> + return ERR_PTR(-EINVAL);
> +
> + return heap->ops->allocate(heap, len, fd_flags, heap_flags);
> +}
> +EXPORT_SYMBOL_NS_GPL(dma_heap_buffer_alloc, "DMA_BUF_HEAP");
>
> - dmabuf = heap->ops->allocate(heap, len, fd_flags, heap_flags);
> +static int dma_heap_bufferfd_alloc(struct dma_heap *heap, size_t len,
> + u32 fd_flags,
> + u64 heap_flags)
> +{
> + struct dma_buf *dmabuf;
> + int fd;
> +
> + dmabuf = dma_heap_buffer_alloc(heap, len, fd_flags, heap_flags);
> if (IS_ERR(dmabuf))
> return PTR_ERR(dmabuf);
>
> @@ -110,15 +133,9 @@ static long dma_heap_ioctl_allocate(struct file *file, void *data)
> if (heap_allocation->fd)
> return -EINVAL;
>
> - if (heap_allocation->fd_flags & ~DMA_HEAP_VALID_FD_FLAGS)
> - return -EINVAL;
> -
> - if (heap_allocation->heap_flags & ~DMA_HEAP_VALID_HEAP_FLAGS)
> - return -EINVAL;
> -
> - fd = dma_heap_buffer_alloc(heap, heap_allocation->len,
> - heap_allocation->fd_flags,
> - heap_allocation->heap_flags);
> + fd = dma_heap_bufferfd_alloc(heap, heap_allocation->len,
> + heap_allocation->fd_flags,
> + heap_allocation->heap_flags);
> if (fd < 0)
> return fd;
>
> @@ -317,6 +334,36 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
> }
> EXPORT_SYMBOL_NS_GPL(dma_heap_add, "DMA_BUF_HEAP");
>
> +/**
> + * dma_heap_find - get the heap registered with the specified name
> + * @name: Name of the DMA-Heap to find
> + *
> + * Returns:
> + * The DMA-Heap with the provided name.
> + *
> + * NOTE: DMA-Heaps returned from this function MUST be released using
> + * dma_heap_put() when the user is done to enable the heap to be unloaded.
> + */
> +struct dma_heap *dma_heap_find(const char *name)

s/dma_heap_find/dma_heap_find_by_name/gc

> +{
> + struct dma_heap *h;
> +
> + mutex_lock(&heap_list_lock);
> + list_for_each_entry(h, &heap_list, list) {
> + if (!kref_get_unless_zero(&h->refcount))
> + continue;
> +
> + if (!strcmp(h->name, name)) {

I think we should go sysfs_streq(), to make sure we don't return a heap
whose name only starts with the searched name.

> + mutex_unlock(&heap_list_lock);
> + return h;
> + }
> + dma_heap_put(h);
> + }
> + mutex_unlock(&heap_list_lock);
> + return NULL;

This could be simplified with something like:

struct dma_heap *h, *ret = NULL;

guard(mutex)(&heap_list_lock);
list_for_each_entry(h, &heap_list, list) {
if (!sysfs_streq(h->name, name))
continue;

if (kref_get_unless_zero(&h->refcount))
ret = h;

break;
}

return ret;

> +}

> +EXPORT_SYMBOL_NS_GPL(dma_heap_find, "DMA_BUF_HEAP");
> +
> static void dma_heap_release(struct kref *ref)
> {
> struct dma_heap *heap = container_of(ref, struct dma_heap, refcount);
> @@ -341,6 +388,7 @@ void dma_heap_put(struct dma_heap *heap)
> {
> kref_put(&heap->refcount, dma_heap_release);
> }
> +EXPORT_SYMBOL_NS_GPL(dma_heap_put, "DMA_BUF_HEAP");
>
> static char *dma_heap_devnode(const struct device *dev, umode_t *mode)
> {
> diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
> index ff57741700f5f..c3351f8a1f8cf 100644
> --- a/include/linux/dma-heap.h
> +++ b/include/linux/dma-heap.h
> @@ -46,8 +46,14 @@ const char *dma_heap_get_name(struct dma_heap *heap);
>
> struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info);
>
> +struct dma_heap *dma_heap_find(const char *name);
> +
> void dma_heap_put(struct dma_heap *heap);
>
> +struct dma_buf *dma_heap_buffer_alloc(struct dma_heap *heap, size_t len,
> + u32 fd_flags,
> + u64 heap_flags);
> +
> extern bool mem_accounting;
>
> #endif /* _DMA_HEAPS_H */