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

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Date: Tue, 4 Jun 2024 20:42:55 +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 Tue, Jun 04, 2024 at 02:39:18PM -0500, David Lechner wrote:
> On 6/4/24 2:33 PM, Conor Dooley wrote:
> > 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.
> 
> I was more ignorant back then. :-)
> 
> That is is why I said "thought" instead of "think". I am more enlightened now.

Heh, I didn't mean it in a bad way. I just wanted to flesh out why you
thought that way.


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