[PATCH v2 02/12] iio: buffer-dma: Enable buffer write support

[Nuno Sá]

On Mon, 2022-03-28 at 19:39 +0100, Paul Cercueil wrote:
> Hi Jonathan,
> 
> Le lun., mars 28 2022 at 18:24:09 +0100, Jonathan Cameron 
> <jic23 at kernel.org> a écrit :
> > On Mon,  7 Feb 2022 12:59:23 +0000
> > Paul Cercueil <paul at crapouillou.net> wrote:
> > 
> > >  Adding write support to the buffer-dma code is easy - the
> > > write()
> > >  function basically needs to do the exact same thing as the
> > > read()
> > >  function: dequeue a block, read or write the data, enqueue the
> > > block
> > >  when entirely processed.
> > > 
> > >  Therefore, the iio_buffer_dma_read() and the new 
> > > iio_buffer_dma_write()
> > >  now both call a function iio_buffer_dma_io(), which will perform
> > > this
> > >  task.
> > > 
> > >  The .space_available() callback can return the exact same value
> > > as 
> > > the
> > >  .data_available() callback for input buffers, since in both
> > > cases we
> > >  count the exact same thing (the number of bytes in each
> > > available
> > >  block).
> > > 
> > >  Note that we preemptively reset block->bytes_used to the
> > > buffer's 
> > > size
> > >  in iio_dma_buffer_request_update(), as in the future the
> > >  iio_dma_buffer_enqueue() function won't reset it.
> > > 
> > >  v2: - Fix block->state not being reset in
> > >        iio_dma_buffer_request_update() for output buffers.
> > >      - Only update block->bytes_used once and add a comment about
> > > why we
> > >        update it.
> > >      - Add a comment about why we're setting a different state
> > > for 
> > > output
> > >        buffers in iio_dma_buffer_request_update()
> > >      - Remove useless cast to bool (!!) in iio_dma_buffer_io()
> > > 
> > >  Signed-off-by: Paul Cercueil <paul at crapouillou.net>
> > >  Reviewed-by: Alexandru Ardelean <ardeleanalex at gmail.com>
> > One comment inline.
> > 
> > I'd be tempted to queue this up with that fixed, but do we have
> > any users?  Even though it's trivial I'm not that keen on code
> > upstream well in advance of it being used.
> 
> There's a userspace user in libiio. On the kernel side we do have 
> drivers that use it in ADI's downstream kernel, that we plan to 
> upstream in the long term (but it can take some time, as we need to 
> upstream other things first, like JESD204B support).
> 
> 

You mean, users for DMA output buffers? If so, I have on my queue to
add the dac counterpart of this one:

https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/adi-axi-adc.c

Which is a user of DMA buffers. Though this one does not depend on
JESD204, I suspect it will also be a tricky process mainly because I
think there are major issues on how things are done right now (on the
ADC driver).

But yeah, not a topic here and I do plan to first start the discussion
on the mailing list before starting developing (hopefully in the coming
weeks)...

- Nuno Sá