Re: [PATCH RFC v2 8/8] iio: adc: ad7944: add support for SPI offload

[Jonathan Cameron <jic23@xxxxxxxxxx>]

On Sat, 11 May 2024 13:41:09 -0500
David Lechner <dlechner@xxxxxxxxxxxx> wrote:

> On Sat, May 11, 2024 at 11:58 AM Jonathan Cameron <jic23@xxxxxxxxxx> wrote:
> >
> > On Fri, 10 May 2024 19:44:31 -0500
> > David Lechner <dlechner@xxxxxxxxxxxx> wrote:
> >  
> > > This adds support for SPI offload to the ad7944 driver. This allows
> > > reading data at the max sample rate of 2.5 MSPS.
> > >
> > > Signed-off-by: David Lechner <dlechner@xxxxxxxxxxxx>
> > > ---
> > >
> > > v2 changes:
> > >
> > > In the previous version, there was a new separate driver for the PWM
> > > trigger and DMA hardware buffer. This was deemed too complex so they
> > > are moved into the ad7944 driver.
> > >
> > > It has also been reworked to accommodate for the changes described in
> > > the other patches.
> > >
> > > RFC: This isn't very polished yet, just FYI. A few things to sort out:
> > >
> > > Rather than making the buffer either triggered buffer or hardware buffer,
> > > I'm considering allowing both, e.g. buffer0 will always be the triggered
> > > buffer and buffer1 will will be the hardware buffer if connected to a SPI
> > > controller with offload support, otherwise buffer1 is absent. But since
> > > multiple buffers haven't been used much so far, more investigation is
> > > needed to see how that would work in practice. If we do that though, then
> > > we would always have the sampling_frequency attribute though even though
> > > it only applies to one buffer.  
> >
> > Why would someone who has this nice IP in the path want the conventional
> > triggered buffer?  I'm not against the two buffer option, but I'd like to know
> > the reasoning not to just provide the hardware buffer if this SPI offload
> > is available.
> >
> > I can conjecture reasons but would like you to write them out for me :)
> > This feels like if someone has paid for the expensive hardware they probably
> > only want the best performance.
> >  
> 
> For me, it was more of a question of if we need to keep the userspace
> interface consistent between both with or without offload support. But
> if you are happy with it this way where we have only one or the other,
> it is less work for me. :-)

So inconsistency in userspace interfaces can occur for many reasons like
whether the interrupt is wired or not, but in this particularly
case I guess we have ABI stability issue because there are boards out there
today and people using the driver without this offload functionality.
I'd not really thought that bit through, so I think you are correct that
we need to maintain the triggered buffer interface and 'add' the new
ABI for the offloaded case.  The multibuffer approach should work for this.
Will be interesting if any problem surface from having two very different
types of buffer on the same device.

Jonathan