Re: [PATCH v2 1/9] dt-bindings: iio: dac: ad3552r: add io-backend property

["Nuno Sá" <noname.nuno@gmail.com>]

On Mon, 2024-09-09 at 12:19 -0500, David Lechner wrote:
> On 9/9/24 9:03 AM, Nuno Sá wrote:
> > On Mon, 2024-09-09 at 13:46 +0100, Conor Dooley wrote:
> > > On Sun, Sep 08, 2024 at 01:29:25PM +0100, Jonathan Cameron wrote:
> > > > On Thu, 05 Sep 2024 17:17:31 +0200
> > > > Angelo Dureghello <adureghello@baylibre.com> wrote:
> > > > 
> > > > > From: Angelo Dureghello <adureghello@baylibre.com>
> > > > > 
> > > > > There is a version AXI DAC IP block (for FPGAs) that provides
> > > > > a physical bus for AD3552R and similar chips. This can be used
> > > > > instead of a typical SPI controller to be able to use the chip
> > > > > in ways that typical SPI controllers are not capable of.
> > > > > 
> > > > > The binding is modified so that either the device is a SPI
> > > > > peripheral or it uses an io-backend.
> > > > > 
> > > > > Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
> > > > 
> > > > >  
> > > > >  required:
> > > > >    - compatible
> > > > > -  - reg
> > > > > -  - spi-max-frequency
> > > > Sort of feels like both reg and spi-max-frequency
> > > > are valid things to specify. 
> > > > 
> > > > Maybe we have an excellent IP and dodgy wiring so want
> > > > to clamp the frequency (long term - don't need to support
> > > > in the driver today).
> > > > 
> > > > Maybe we have an axi_dac IP that supports multiple
> > > > front end devices?  So maybe just keep reg?
> > > 
> > > I'd like to be convinced that this incarnation of the AXI DAC IP is not
> > > a spi controller and that a ref to spi-controller.yaml is not out of
> > > place here. It may not be something that you'd ever use generally, given
> > > the "weird" interface to it, but it does seem to be one regardless.
> > > 
> > 
> > Agreed.. As weird as it get's, it's acting as a spi controller.
> > 
> > > I'd also really like to know how this fits in with spi-offloads. It
> > > /feels/, and I'd like to reiterate the word feels, like a rather similar
> > > idea just applied to a DAC instead of an ADC.
> > 
> > The offload main principle is to replay a spi transfer periodically given an
> > input trigger. I'm not so sure we have that same principle in here. In here
> > I
> > guess we stream data over the qspi interface based on SCLK which can look
> > similar. The difference is that this IP does not need any trigger for any
> > spi
> > transfer replay (I think). 
> > 
> 
> Looking at the AD3552R from a SPI offload perspective of triggered SPI
> messages, I think it still works.
> 
> The trigger doesn't have to be a clock/PWM. In this case, the trigger would
> be whenever the IIO buffer is full and ready to send a burst of data (not
> sure if this would be a hardware or software trigger - but it works either
> way).
> 

Right... That's what we already have for DACs with HW buffering.

> Also, the DAC_CUSTOM_CTRL::ADDRESS register field in the AXI DAC IP core
> acts as an offload to record and play back a SPI write transfer.
> 
> If we were using the AXI SPI Engine, this would be one SPI message with
> two xfers, one for the address write followed by one for the data write.

Just a nipick comment. At least from the current implementation the address is
only writen once before starting to stream. So I guess we would not want to
replay that xfer for every sample.

> The size of the data write would be the size of the IIO buffer - or in
> the case of a cyclic DMA, the size of the write data would be channel
> data size * num channels and the xfer would have a special cyclic offload
> flag set.
> 
> So I think we could make a single binding that works for the the AXI DAC
> backend/offload and the AXI SPI Engine offload. (I don't think it would
> be so easy to integrate the AXI DAC into the SPI framework on the driver
> side - and hopefully we won't have to, but the DT still could use the
> proposed SPI offload bindings.)
> 

Hopefully not... 


- Nuno Sá