Date: Tue, 4 Jun 2024 20:33:09 +0100
From: Conor Dooley <conor@...nel.org>
To: David Lechner <dlechner@...libre.com>
Cc: Nuno Sá <noname.nuno@...il.com>,
	Mark Brown <broonie@...nel.org>,
	Jonathan Cameron <jic23@...nel.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Nuno Sá <nuno.sa@...log.com>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	David Jander <david@...tonic.nl>,
	Martin Sperl <kernel@...tin.sperl.org>, linux-spi@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-iio@...r.kernel.org
Subject: Re: [PATCH RFC v2 1/8] spi: dt-bindings: spi-peripheral-props: add
 spi-offloads property

On Thu, May 30, 2024 at 02:24:17PM -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.
> 
> > 
> > Not familiar enough with of_graph so can't argue about it but likely is something
> > worth looking at.
> 
> 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.

Could you elaborate on why you think you can assume the connections? What
happens when you have multiple stages of data processing and/or multiple
ADCs in your system? As I've previously said, I work on FPGA stuff, and
everyone here seems to fawn over having <insert custom DSP IP here> in
their data pipelines. I can't imagine it being any different for ADC data,
and an io-backend property that doesn't describe how the data flows is
gonna become lacklustre I think.

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