Re: [PATCH v2 0/3] iio: buffer: Fix DMABUF mapping in some systems

["Nuno Sá" <noname.nuno@gmail.com>]

On Mon, 2025-10-06 at 11:18 -0500, David Lechner wrote:
> On 10/6/25 11:06 AM, Nuno Sá via B4 Relay 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 v2:
> > - Dropped Fixes tags on the first two patches and Cc stable them instead
> >   (as prerequisites for the third patch). 
> > - Link to v1:
> > https://lore.kernel.org/r/20251002-fix-iio-dmabuf-get-dma-device-v1-0-c1c9945029d0@analog.com
> 
> Did you not care for my other suggestions in v1?

Completely missed them, sorry! I kind of stop reading in the stable stuff. I'm
ok with the helper function. For the clarification I feel it's redundant. The
field is called .get_dma_dev() and the description "called to get the DMA
channel associated with this buffer" already implies is for DMA buffer. Same as
ops like .enqueue_dmabuf().

- Nuno Sá