linux-kernel - Re: [PATCH 05/13] spi: axi-spi-engine: add SPI offload support

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Message-ID: <CAMknhBEEC4F2_hpJ_405bfrb3KNkAYpjDoJbnmOFXodp8yLACg@mail.gmail.com>
Date: Wed, 10 Jan 2024 16:31:25 -0600
From: David Lechner <dlechner@...libre.com>
To: Mark Brown <broonie@...nel.org>
Cc: Jonathan Cameron <jic23@...nel.org>, Rob Herring <robh+dt@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	Michael Hennerich <michael.hennerich@...log.com>, Nuno Sá <nuno.sa@...log.com>, 
	Frank Rowand <frowand.list@...il.com>, Thierry Reding <thierry.reding@...il.com>, 
	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>, 
	Jonathan Corbet <corbet@....net>, linux-spi@...r.kernel.org, linux-iio@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Lars-Peter Clausen <lars@...afoo.de>
Subject: Re: [PATCH 05/13] spi: axi-spi-engine: add SPI offload support

On Wed, Jan 10, 2024 at 3:39 PM Mark Brown <broonie@...nel.org> wrote:
>
> On Wed, Jan 10, 2024 at 01:49:46PM -0600, David Lechner wrote:
> > This adds an implementation of the SPI offload_ops to the AXI SPI Engine
> > driver to provide offload support.
> >
> > Offload lookup is done by device property lookup. SPI Engine commands
> > and tx data  are recorded by writing to offload-specific FIFOs in the
> > SPI Engine hardware.
>
> Glancing through here I'm not seeing anything here that handles DMA
> mapping, given that the controller will clearly be doing DMA here that
> seems surprising.

In the use case implemented in this series, the RX data is going to
DMA, but in general, that doesn't have to be the case. In theory, it
could get piped directly to a DSP or something like that. So I left
the RX DMA part out of the SPI controller and implemented as a
separate device in "iio: offload: add new PWM triggered DMA buffer
driver". The SPI controller itself isn't aware that it is connected to
DMA (i.e. there are no registers that have to be poked to enable DMA
or anything like that).