Re: [PATCH v2 11/15] accel/qda: Add PRIME DMA-BUF import support

[Dmitry Baryshkov]

On Mon, Aug 17, 2026 at 10:17:46AM +0530, Ekansh Gupta wrote:
> Allow user-space to import DMA-BUF file descriptors from other
> subsystems (GPU, camera, video) into the QDA driver via the standard
> DRM PRIME interface.
> 
> qda_prime.c
>   Implements qda_gem_prime_import(), which is set as the driver's
>   .gem_prime_import callback. On import it:
>   1. Short-circuits self-import: if the dma_buf was exported by this
>      device and is not itself an import, the existing GEM object is
>      returned with an incremented reference count.
>   2. Attaches to the dma_buf and maps it with DMA_BIDIRECTIONAL via
>      dma_buf_map_attachment_unlocked(), obtaining an sg_table whose
>      DMA addresses are IOMMU virtual addresses in the CB device's
>      address space.
>   3. Calls qda_memory_manager_alloc() to record the mapping and encode
>      the SID in the upper bits of the DMA address, matching the
>      convention used for natively allocated buffers.
> 
>   qda_prime_fd_to_handle() wraps drm_gem_prime_fd_to_handle() under
>   qdev->import_lock, storing the calling file_priv in

Why do you need a separate lock?

>   qdev->current_import_file_priv so that qda_gem_prime_import() can
>   retrieve it (the .gem_prime_import callback does not receive
>   file_priv directly, but the context bank to attach to is per-process).
> 
> qda_gem.c
>   qda_gem_free_object() is extended to handle the imported-buffer
>   teardown path: unmap the sg_table, detach from the dma_buf, and
>   release the dma_buf reference.
>   qda_gem_mmap_obj() rejects mmap requests on imported objects, which
>   must be mapped through the exporter instead.
> 
> qda_memory_manager.c
>   The DSP is given a single base address per buffer, so only buffers
>   that are mapped as one contiguous range can be described to it.

Is there no IOMMU in front of the DSP? Can we remap the buffer to a
linear are inside the DSP map?

>   qda_memory_manager_map_imported() therefore walks the imported
>   buffer's scatterlist and rejects any buffer whose entries are not
>   contiguous; contiguous imports (for example from the CMA DMA-buf
>   heap) are accepted. Whether an exporter or IOMMU coalesces a buffer
>   into a single range is not guaranteed, so this is verified rather
>   than assumed.
>   qda_memory_manager_free() skips the DMA free path for imported
>   buffers since the memory is owned by the exporter.
> 
> Assisted-by: Claude:claude-sonnet-5
> Signed-off-by: Ekansh Gupta <ekansh.gupta@xxxxxxxxxxxxxxxx>
> ---
> Changes in v2:
> - Do not assume the exporter or IOMMU maps an imported buffer as a
>   single contiguous range; walk the scatterlist and cleanly reject
>   non-contiguous imports instead (Christian König)
> - Document that only contiguous imports (e.g. CMA DMA-buf heap) are
>   supported
> - Return -EBUSY rather than -ENODEV when no context bank is available
> ---
>  drivers/accel/qda/Makefile             |   1 +
>  }
>  
> +static int qda_memory_manager_map_imported(struct qda_gem_obj *gem_obj,
> +					   struct qda_iommu_device *iommu_dev)
> +{
> +	dma_addr_t dma_addr, expected_addr = 0;
> +	struct scatterlist *sg, *s;
> +	int i;
> +
> +	sg = gem_obj->sgt->sgl;
> +
> +	/*
> +	 * Verify that the imported DMA-BUF is contiguous. While some platforms

Which "some platforms"?

> +	 * or IOMMUs might map buffers as a single range, we must not silently
> +	 * assume this is always the case. We loop through the scatterlist
> +	 * entries and reject any that are non-contiguous.
> +	 */

-- 
With best wishes
Dmitry