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

[Paul Cercueil <paul@crapouillou.net>]

Hi Jonathan,

Le jeudi 21 décembre 2023 à 16:12 +0000, Jonathan Cameron a écrit :
> 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.

The length of the sglist will always be enough, the
iio_buffer_enqueue_dmabuf() function already checks that block-
>bytes_used is equal or smaller than the size of the DMABUF.

It is quite a few functions above in the call stack though, so I can
handle the errors of sg_nents_for_len() here if you think makes sense.

Cheers,
-Paul

> > +
> > +		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;
> >  
>