Re: [PATCH v3 0/3] iio: buffer: Fix DMABUF mapping in some systems

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 07 Oct 2025 10:15:20 +0100
Nuno Sá via B4 Relay <devnull+nuno.sa.analog.com@kernel.org> wrote:

> This series fixes an issue with DMABUF support in the IIO subsystem where
> the wrong DMA device could be used for buffer mapping operations. This
> becomes critical on systems like Xilinx/AMD ZynqMP Ultrascale where memory
> can be mapped above the 32-bit address range.
> 
> Problem:
> --------
> The current IIO DMABUF implementation assumes it can use the parent device
> of the IIO device for DMA operations. However, this device may not have
> the appropriate DMA mask configuration for accessing high memory addresses.
> On systems where memory is mapped above 32-bits, this leads to the use of
> bounce buffers through swiotlb, significantly impacting performance.
> 
> Solution:
> ---------
> This series introduces a new .get_dma_dev() callback in the buffer access
> functions that allows buffer implementations to specify the correct DMA
> device that should be used for DMABUF operations. The DMA buffer
> infrastructure implements this callback to return the device that actually
> owns the DMA channel, ensuring proper memory mapping without bounce buffers.
> 
> Changes:
> --------
> 1. Add .get_dma_dev() callback to iio_buffer_access_funcs and update core
>    DMABUF code to use it when available
> 2. Implement the callback in the DMA buffer infrastructure
> 3. Wire up the callback in the dmaengine buffer implementation
> 
> This ensures that DMABUF operations use the device with the correct DMA
> configuration, eliminating unnecessary bounce buffer usage and improving
> performance on high-memory systems.
> 
> (AI generated cover. I would not be this formal but I guess is not
> that bad :))
> 
> ---
> Changes in v3:
> - Patch 1
>   * Add a new iio_buffer_get_dma_dev() helper to get the DMA dev.
> - Link to v2: https://lore.kernel.org/r/20251006-fix-iio-dmabuf-get-dma-device-v2-0-d960bc9084da@analog.com

Looks like I misinterpreted outcome of v2 thread.  Anyhow dropped that and picked
this up.

Thanks

J
> 
> ---
> Nuno Sá (3):
>       iio: buffer: support getting dma channel from the buffer
>       iio: buffer-dma: support getting the DMA channel
>       iio: buffer-dmaengine: enable .get_dma_dev()
> 
>  drivers/iio/buffer/industrialio-buffer-dma.c       |  6 ++++++
>  drivers/iio/buffer/industrialio-buffer-dmaengine.c |  2 ++
>  drivers/iio/industrialio-buffer.c                  | 21 ++++++++++++++++-----
>  include/linux/iio/buffer-dma.h                     |  1 +
>  include/linux/iio/buffer_impl.h                    |  2 ++
>  5 files changed, 27 insertions(+), 5 deletions(-)
> ---
> base-commit: b9700f87939f0f477e5c00db817f54ab8a97702b
> change-id: 20250930-fix-iio-dmabuf-get-dma-device-339ac70543db
> --
> 
> Thanks!
> - Nuno Sá
> 
> 
>