Re: [PATCH RFC v2 1/8] spi: dt-bindings: spi-peripheral-props: add spi-offloads property

["Nuno Sá" <noname.nuno@gmail.com>]

On Thu, 2024-05-30 at 14:24 -0500, David Lechner wrote:
> On 5/29/24 3:07 AM, Nuno Sá wrote:
> > On Sun, 2024-05-26 at 18:35 +0100, Conor Dooley wrote:
> 
> 
> > > It might be easy to do it this way right now, but be problematic for a
> > > future device or if someone wants to chuck away the ADI provided RTL and
> > > do their own thing for this device. Really it just makes me wonder if
> > > what's needed to describe more complex data pipelines uses an of_graph,
> > > just like how video pipelines are handled, rather than the implementation
> > > of io-backends that don't really seem to model the flow of data.
> > > 
> > 
> > Yeah, backends is more for devices/soft-cores that extend the functionality of
> > the
> > device they are connected too. Like having DACs/ADCs hdl cores for connecting to
> > high
> > speed controllers. Note that in some cases they also manipulate or even create
> > data
> > but since they fit in IIO, having things like the DMA property in the hdl binding
> > was
> > fairly straight.
> > 
> > Maybe having an offload dedicated API (through spi) to get/share a DMA handle
> > would
> > be acceptable. Then we could add support to "import" it in the IIO core. Then it
> > would be up to the controller to accept or not to share the handle (in some cases
> > the
> > controller could really want to have the control of the DMA transfers).
> 
> I could see this working for some SPI controllers, but for the AXI SPI Engine
> + DMA currently, the DMA has a fixed word size, so can't be used as a generic
> DMA with arbitrary SPI xfers. For example, if the HDL is compiled with a 32-bit
> word size, then even if we are reading 16-bit sample data, the DMA is going to
> put it in a 32-bit slot. So one could argue that this is still doing some data
> manipulation similar to the CRC checker example.
> 

Note that what I was thinking was something very trivial... Just a way to get
'dma_chan' out of the spi_engine to the consumer so we could import it in the IIO DMA
infrastructure... I assume it may be a sneaky way to just get around the problem
though :).

Another way is to come up with spi like API's to submit DMA request's (passing an
handler or so for completion). I guess we would somehow also need a kind of get()
function that would give consumers some kind of info/interface like spi xfers size
(as in this particular case it's the DMA who defines the src/dst width). We would
likely also need some kind of spi_dma_buffer implementation in IIO (likely to share
some code with the current stuff; at least the userspace interface should definitely
be the same).

Anyways, the above it's just me throwing some random ideas that come to mind :). They
may be stupid but at the very least they could give you some betters ideas :).

> > 
> > Not familiar enough with of_graph so can't argue about it but likely is something
> > worth looking at.
> > 
> > - Nuno Sá
> > > > 
> 
> I did try implementing something using graph bindings when I first started
> working on this, but it didn't seem to really give us any extra useful
> information. It was just describing connections (endpoints) that I thought
> we could just implicitly assume. After this discussion though, maybe worth
> a second look. I'll have to think about it more.

Why not :)?

- Nuno Sá