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

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Message-ID: <277071605eb355912972a30b07ecead7d70efe25.camel@gmail.com>
Date: Fri, 22 Dec 2023 09:58:29 +0100
From: Nuno Sá <noname.nuno@...il.com>
To: Paul Cercueil <paul@...pouillou.net>, Jonathan Cameron <jic23@...nel.org>
Cc: Michael Hennerich <Michael.Hennerich@...log.com>, Jonathan Corbet
	 <corbet@....net>, linux-iio@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, Christian
	König
	 <christian.koenig@....com>, linaro-mm-sig@...ts.linaro.org, Vinod Koul
	 <vkoul@...nel.org>, dmaengine@...r.kernel.org, Sumit Semwal
	 <sumit.semwal@...aro.org>, linux-media@...r.kernel.org
Subject: Re: [PATCH v5 7/8] iio: buffer-dmaengine: Support new DMABUF based
 userspace API

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@...pouillou.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@...pouillou.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?

- Nuno Sá