Re: [PATCH v5 7/8] iio: buffer-dmaengine: Support new DMABUF based userspace API

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 19 Dec 2023 18:50:08 +0100
Paul Cercueil <paul@crapouillou.net> wrote:

> Use the functions provided by the buffer-dma core to implement the
> DMABUF userspace API in the buffer-dmaengine IIO buffer implementation.
> 
> Since we want to be able to transfer an arbitrary number of bytes and
> not necesarily the full DMABUF, the associated scatterlist is converted
> to an array of DMA addresses + lengths, which is then passed to
> dmaengine_prep_slave_dma_array().
> 
> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
One question inline. Otherwise looks fine to me.

J
> 
> ---
> v3: Use the new dmaengine_prep_slave_dma_array(), and adapt the code to
>     work with the new functions introduced in industrialio-buffer-dma.c.
> 
> v5: - Use the new dmaengine_prep_slave_dma_vec().
>     - Restrict to input buffers, since output buffers are not yet
>       supported by IIO buffers.
> ---
>  .../buffer/industrialio-buffer-dmaengine.c    | 52 ++++++++++++++++---
>  1 file changed, 46 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/iio/buffer/industrialio-buffer-dmaengine.c b/drivers/iio/buffer/industrialio-buffer-dmaengine.c
> index 5f85ba38e6f6..825d76a24a67 100644
> --- a/drivers/iio/buffer/industrialio-buffer-dmaengine.c
> +++ b/drivers/iio/buffer/industrialio-buffer-dmaengine.c
> @@ -64,15 +64,51 @@ static int iio_dmaengine_buffer_submit_block(struct iio_dma_buffer_queue *queue,
>  	struct dmaengine_buffer *dmaengine_buffer =
>  		iio_buffer_to_dmaengine_buffer(&queue->buffer);
>  	struct dma_async_tx_descriptor *desc;
> +	unsigned int i, nents;
> +	struct scatterlist *sgl;
> +	struct dma_vec *vecs;
> +	size_t max_size;
>  	dma_cookie_t cookie;
> +	size_t len_total;
>  
> -	block->bytes_used = min(block->size, dmaengine_buffer->max_size);
> -	block->bytes_used = round_down(block->bytes_used,
> -			dmaengine_buffer->align);
> +	if (queue->buffer.direction != IIO_BUFFER_DIRECTION_IN) {
> +		/* We do not yet support output buffers. */
> +		return -EINVAL;
> +	}
>  
> -	desc = dmaengine_prep_slave_single(dmaengine_buffer->chan,
> -		block->phys_addr, block->bytes_used, DMA_DEV_TO_MEM,
> -		DMA_PREP_INTERRUPT);
> +	if (block->sg_table) {
> +		sgl = block->sg_table->sgl;
> +		nents = sg_nents_for_len(sgl, block->bytes_used);

Are we guaranteed the length in the sglist is enough?  If not this
can return an error code.


> +
> +		vecs = kmalloc_array(nents, sizeof(*vecs), GFP_KERNEL);
> +		if (!vecs)
> +			return -ENOMEM;
> +
> +		len_total = block->bytes_used;
> +
> +		for (i = 0; i < nents; i++) {
> +			vecs[i].addr = sg_dma_address(sgl);
> +			vecs[i].len = min(sg_dma_len(sgl), len_total);
> +			len_total -= vecs[i].len;
> +
> +			sgl = sg_next(sgl);
> +		}
> +
> +		desc = dmaengine_prep_slave_dma_vec(dmaengine_buffer->chan,
> +						    vecs, nents, DMA_DEV_TO_MEM,
> +						    DMA_PREP_INTERRUPT);
> +		kfree(vecs);
> +	} else {
> +		max_size = min(block->size, dmaengine_buffer->max_size);
> +		max_size = round_down(max_size, dmaengine_buffer->align);
> +		block->bytes_used = max_size;
> +
> +		desc = dmaengine_prep_slave_single(dmaengine_buffer->chan,
> +						   block->phys_addr,
> +						   block->bytes_used,
> +						   DMA_DEV_TO_MEM,
> +						   DMA_PREP_INTERRUPT);
> +	}
>  	if (!desc)
>  		return -ENOMEM;
>