linux-kernel - Re: [PATCH 01/13] spi: add core support for controllers with offload capabilities

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Message-ID: <829ac770-1955-45b7-9033-6ed60ffdf77e@sirena.org.uk>
Date: Thu, 11 Jan 2024 21:49:53 +0000
From: Mark Brown <broonie@...nel.org>
To: David Lechner <dlechner@...libre.com>
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, David Jander <david@...tonic.nl>
Subject: Re: [PATCH 01/13] spi: add core support for controllers with offload
 capabilities

On Thu, Jan 11, 2024 at 03:32:54PM -0600, David Lechner wrote:
> On Thu, Jan 11, 2024 at 2:54 PM David Lechner <dlechner@...libre.com> wrote:

> > > (CCed) a while back when he was doing all the work he did on optimising
> > > the core for uncontended uses, the thinking there was to have a
> > > spi_prepare_message() (or similar) API that drivers could call and then
> > > reuse the same transfer repeatedly, and even without any interface for
> > > client drivers it's likely that we'd be able to take advantage of it in
> > > the core for multi-transfer messages.  I'd be surprised if there weren't
> > > wins when the message goes over the DMA copybreak size.  A much wider
> > > range of hardware would be able to do this bit, for example David's case
> > > was a Raspberry Pi using the DMA controller to write into the SPI

> For those, following along, it looks like the RPi business was
> actually a 2013 discussion with Martin Sperl [2]. Both this and [1]
> discuss proposed spi_prepare_message() APIs.

> [2]: https://lore.kernel.org/linux-spi/CACRpkdb4mn_Hxg=3tuBu89n6eyJ082EETkwtNbzZDFZYTHbVVg@mail.gmail.com/T/#u

Oh, yes - sorry, I'd misremembered which optimisation effort it was
associated with.  Apologies.

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