linux-kernel - Re: [PATCH 3/6] spi: add multi_bus_mode field to struct spi

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Message-ID: <8c7bf62a-c5dc-4e4d-8059-8abea15ba94e@sirena.org.uk>
Date: Wed, 15 Oct 2025 16:18:06 +0100
From: Mark Brown <broonie@...nel.org>
To: Nuno Sá <noname.nuno@...il.com>
Cc: David Lechner <dlechner@...libre.com>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Marcelo Schmitt <marcelo.schmitt@...log.com>,
	Michael Hennerich <michael.hennerich@...log.com>,
	Nuno Sá <nuno.sa@...log.com>,
	Jonathan Cameron <jic23@...nel.org>,
	Andy Shevchenko <andy@...nel.org>,
	Sean Anderson <sean.anderson@...ux.dev>, linux-spi@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-iio@...r.kernel.org
Subject: Re: [PATCH 3/6] spi: add multi_bus_mode field to struct spi_transfer

On Wed, Oct 15, 2025 at 03:43:09PM +0100, Nuno Sá wrote:
> On Wed, 2025-10-15 at 13:01 +0100, Mark Brown wrote:
> > On Wed, Oct 15, 2025 at 11:16:01AM +0100, Nuno Sá wrote:
> > > On Tue, 2025-10-14 at 17:02 -0500, David Lechner wrote:

> > > >         controller    < data bits <     peripheral
> > > >         ----------   ----------------   ----------
> > > >             SDI 0    0-0-0-1-0-0-0-1    SDO 0
> > > >             SDI 1    1-0-0-0-1-0-0-0    SDO 1

> > > Out of curiosity, how does this work for devices like AD4030 where the same
> > > word
> > > is kind of interleaved between SDO lines? I guess it works the same (in
> > > terms of
> > > SW) and is up to some IP core (typically in the FPGA) to "re-assemble" the
> > > word?

> > So combined with the existing parallel SPI support?

> Not sure if this is meant for me :). parallel SPI is for parallel memories and
> the spi_device multi cs support stuff right? I tried to track it down but it's
> not clear if there are any users already upstream (qspi zynqmp and the nor
> flashes). It looks like it's not in yet but not sure.

There's multi-CS stuff but what I was thinking about was the stuff for
parallel memories, I was trying to clarify what cases you were talking
about with "interleaved between SDO lines".

> Anyways, IIUC, it seems we could indeed see the device I mentioned as a parallel
> kind of thing as we have one bit per lane per sclk. However, the multi_cs
> concept does not apply (so I think it would be misleading to try and hack it
> around with tweaking cs_index_mask and related APIs).

OK, so either just the parallel SPI or possibly that composed with this
(fun!).

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