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

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 22 Dec 2023 09:58:29 +0100
Nuno Sá <noname.nuno@gmail.com> wrote:

> On Thu, 2023-12-21 at 18:30 +0100, Paul Cercueil wrote:
> > 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.  
> 
> Maybe putting something like the above in a comment?
Either comment, or an explicit check so we don't need the comment is
fine as far as I'm concerned.

Jonathan

> 
> - Nuno Sá
> 
>