Re: [PATCH 3/3] uio: Add UIO_DMABUF_HEAP

Nicolas Dufresne
2025-04-11T18:41:13Z

Hi Bastien,

Le jeudi 10 avril 2025 à 16:53 +0200, Bastien Curutchet a écrit :
> Some UIO users need to access DMA addresses from userspace to be able to
> configure DMA done by the UIO device. Currently there is no way of doing
> this.
>
> Add a UIO_DMABUF_HEAP Kconfig option. When selected, a dma-heap
> allocator is created for every new UIO device. This allocator only
> implements 4 basic operations: allocate, release, mmap and get_dma_addr.
> The buffer allocation is done through dma_alloc_coherent().

This is quite redundant with the CMA heap. I believe a better design is
to make UIO devices dmabuf importers. This will make your UIO dmabuf
implementation a lot more useful.

As for the physical addresses, everywhere you currently pass a physical
address, you should be able to add ioctl to pass a DMABuf FD, or a
handle to an UIO specific object (similar to buffer objects in DRM) and
hide these.

regards,
Nicolas

>
> Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
> ---
>  drivers/uio/Kconfig        |   9 ++++
>  drivers/uio/Makefile       |   1 +
>  drivers/uio/uio.c          |   4 ++
>  drivers/uio/uio_heap.c     | 120 +++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/uio_driver.h |   2 +
>  5 files changed, 136 insertions(+)
>
> diff --git a/drivers/uio/Kconfig b/drivers/uio/Kconfig
> index b060dcd7c6350191726c0830a1ae7b9a388ca4bb..2f3b1e57fceb01354b65cc4d39f342f645a238db 100644
> --- a/drivers/uio/Kconfig
> +++ b/drivers/uio/Kconfig
> @@ -164,4 +164,13 @@ config UIO_DFL
>       opae-sdk/tools/libopaeuio/
>
>     If you compile this as a module, it will be called uio_dfl.
> +
> +config UIO_DMABUF_HEAP
> + bool "DMA-BUF UIO Heap"
> + select DMABUF_HEAPS
> + help
> +   Choose this option to enable DMA-BUF UIO heap. It will create a new
> +   heap allocator under /dev/dma_heap/ for every UIO device. This
> +   allocator allows userspace applications to allocate DMA buffers and
> +   access their DMA addresses thanks to the DMA_BUF_IOCTL_GET_DMA_HANDLE
>  endif
> diff --git a/drivers/uio/Makefile b/drivers/uio/Makefile
> index 1c5f3b5a95cf5b681a843b745a046d7ce123255d..f6696daa36567a4e5d18b1b89ba688057e758400 100644
> --- a/drivers/uio/Makefile
> +++ b/drivers/uio/Makefile
> @@ -11,3 +11,4 @@ obj-$(CONFIG_UIO_MF624)         += uio_mf624.o
>  obj-$(CONFIG_UIO_FSL_ELBC_GPCM) += uio_fsl_elbc_gpcm.o
>  obj-$(CONFIG_UIO_HV_GENERIC) += uio_hv_generic.o
>  obj-$(CONFIG_UIO_DFL) += uio_dfl.o
> +obj-$(CONFIG_UIO_DMABUF_HEAP) += uio_heap.o
> diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
> index d93ed4e86a174b5bad193a61aa522cd833fe7bb5..f31936a897805a284165cccfee3d66e96acd4b39 100644
> --- a/drivers/uio/uio.c
> +++ b/drivers/uio/uio.c
> @@ -1046,7 +1046,11 @@ int __uio_register_device(struct module *owner,
>   }
>   }
>
> +#if defined(CONFIG_UIO_DMABUF_HEAP)
> + return add_uio_heap(idev);
> +#else
>   return 0;
> +#endif
>
>  err_request_irq:
>   uio_dev_del_attributes(idev);
> diff --git a/drivers/uio/uio_heap.c b/drivers/uio/uio_heap.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..2e836b503458e280babba0e0adc4f6d8344efc82
> --- /dev/null
> +++ b/drivers/uio/uio_heap.c
> @@ -0,0 +1,120 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/dma-buf.h>
> +#include <linux/dma-heap.h>
> +#include <linux/uio_driver.h>
> +
> +struct uio_heap {
> + struct dma_heap *heap;
> + struct device *dev;
> +};
> +
> +struct uio_heap_buffer {
> + struct uio_heap *heap;
> + dma_addr_t dma_addr;
> + unsigned long len;
> + void *vaddr;
> +};
> +
> +static int uio_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
> +{
> + struct uio_heap_buffer *buffer = dmabuf->priv;
> +
> + return dma_mmap_coherent(buffer->heap->dev, vma, buffer->vaddr,
> + buffer->dma_addr, buffer->len);
> +}
> +
> +static void uio_heap_dma_buf_release(struct dma_buf *dmabuf)
> +{
> + struct uio_heap_buffer *buffer = dmabuf->priv;
> +
> + dma_free_coherent(buffer->heap->dev, buffer->len, buffer->vaddr,
> +   buffer->dma_addr);
> + kfree(buffer);
> +}
> +
> +static int uio_heap_get_dma_addr(struct dma_buf *dmabuf, u64 *addr)
> +{
> + struct uio_heap_buffer *buffer = dmabuf->priv;
> +
> + *addr = buffer->dma_addr;
> + return 0;
> +}
> +
> +static const struct dma_buf_ops uio_heap_buf_ops = {
> + .mmap = uio_heap_mmap,
> + .release = uio_heap_dma_buf_release,
> + .get_dma_addr = uio_heap_get_dma_addr,
> +};
> +
> +static struct dma_buf *uio_heap_allocate(struct dma_heap *heap,
> + unsigned long len,
> + u32 fd_flags,
> + u64 heap_flags)
> +{
> + struct uio_heap *uio_heap = dma_heap_get_drvdata(heap);
> + DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
> + struct uio_heap_buffer *buffer;
> + struct dma_buf *dmabuf;
> +
> + buffer = kzalloc(sizeof(*buffer), GFP_KERNEL);
> + if (!buffer)
> + return ERR_PTR(-ENOMEM);
> +
> + dma_set_coherent_mask(uio_heap->dev, DMA_BIT_MASK(32));
> +
> + buffer->heap = uio_heap;
> + buffer->len = len;
> + buffer->vaddr = dma_alloc_coherent(uio_heap->dev, buffer->len,
> +    &buffer->dma_addr, GFP_KERNEL);
> + if (IS_ERR(buffer->vaddr))
> + goto free_buf;
> +
> + exp_info.exp_name = dma_heap_get_name(heap);
> + exp_info.ops = &uio_heap_buf_ops;
> + exp_info.size = buffer->len;
> + exp_info.flags = fd_flags;
> + exp_info.priv = buffer;
> + dmabuf = dma_buf_export(&exp_info);
> + if (IS_ERR(dmabuf))
> + goto free_dma;
> +
> + return dmabuf;
> +
> +free_dma:
> + dma_free_coherent(uio_heap->dev, buffer->len, buffer->vaddr, buffer->dma_addr);
> +free_buf:
> + kfree(buffer);
> +
> + return ERR_PTR(-ENOMEM);
> +}
> +
> +static const struct dma_heap_ops uio_heap_ops = {
> + .allocate = uio_heap_allocate,
> +};
> +
> +int add_uio_heap(struct uio_device *uio)
> +{
> + struct dma_heap_export_info exp_info;
> + struct uio_heap *uio_heap;
> +
> + uio_heap = kzalloc(sizeof(*uio_heap), GFP_KERNEL);
> + if (!uio_heap)
> + return -ENOMEM;
> +
> + uio_heap->dev = &uio->dev;
> +
> + /* Use device name as heap name */
> + exp_info.name = uio_heap->dev->kobj.name;
> + exp_info.ops = &uio_heap_ops;
> + exp_info.priv = uio_heap;
> +
> + uio_heap->heap = dma_heap_add(&exp_info);
> + if (IS_ERR(uio_heap->heap)) {
> + int ret = PTR_ERR(uio_heap->heap);
> +
> + kfree(uio_heap);
> + return ret;
> + }
> +
> + return 0;
> +}
> diff --git a/include/linux/uio_driver.h b/include/linux/uio_driver.h
> index 18238dc8bfd356a20996ba6cd84f1ff508bbb81c..f8b774d2fa1c7de4b6af881f1e53dfa9f25b3dbf 100644
> --- a/include/linux/uio_driver.h
> +++ b/include/linux/uio_driver.h
> @@ -143,6 +143,8 @@ extern int __must_check
>      struct device *parent,
>      struct uio_info *info);
>
> +extern int add_uio_heap(struct uio_device *uio);
> +
>  /* use a define to avoid include chaining to get THIS_MODULE */
>
>  /**