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

From: Mark Brown <broonie@kernel.org>
To: David Lechner <dlechner@baylibre.com>
Cc: "Jonathan Cameron" <jic23@kernel.org>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Conor Dooley" <conor+dt@kernel.org>,
	"Michael Hennerich" <michael.hennerich@analog.com>,
	"Nuno Sá" <nuno.sa@analog.com>,
	"Frank Rowand" <frowand.list@gmail.com>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Jonathan Corbet" <corbet@lwn.net>,
	linux-spi@vger.kernel.org, linux-iio@vger.kernel.org,
	devicetree@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-pwm@vger.kernel.org, linux-kernel@vger.kernel.org,
	"David Jander" <david@protonic.nl>
Subject: Re: [PATCH 01/13] spi: add core support for controllers with offload capabilities
Date: Thu, 11 Jan 2024 21:49:53 +0000	[thread overview]
Message-ID: <829ac770-1955-45b7-9033-6ed60ffdf77e@sirena.org.uk> (raw)
In-Reply-To: <CAMknhBE-1Khe9J-n5WQnH=mFnN0ukiq7=F-SEOU6J-2_u-R0bw@mail.gmail.com>

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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@baylibre.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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