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

[David Lechner <dlechner@xxxxxxxxxxxx>]

On Thu, Jan 11, 2024 at 3:32 PM David Lechner <dlechner@xxxxxxxxxxxx> wrote:
>
> On Thu, Jan 11, 2024 at 2:54 PM David Lechner <dlechner@xxxxxxxxxxxx> wrote:
> >
> > On Wed, Jan 10, 2024 at 3:36 PM Mark Brown <broonie@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Jan 10, 2024 at 01:49:42PM -0600, David Lechner wrote:
> > > > This adds a feature for specialized SPI controllers that can record
> > > > a series of SPI transfers, including tx data, cs assertions, delays,
> > > > etc. and then play them back using a hardware trigger without CPU
> > > > intervention.
> > >
> > > > The intended use case for this is with the AXI SPI Engine to capture
> > > > data from ADCs at high rates (MSPS) with a stable sample period.
> > >
> > > > Most of the implementation is controller-specific and will be handled by
> > > > drivers that implement the offload_ops callbacks. The API follows a
> > > > prepare/enable pattern that should be familiar to users of the clk
> > > > subsystem.
> > >
> > > This is a lot to do in one go, and I think it's a bit too off on the
> > > side and unintegrated with the core.  There's two very high level bits
> > > here, there's the pre-cooking a message for offloading to be executed by
> > > a hardware engine and there's the bit where that's triggered by some
> > > hardwar event rather than by software.
> > >
> > > There was a bunch of discussion of the former case with David Jander
> >
> > I found [1] which appears to be the conversation you are referring to.
> > Is that all or is there more that I missed?
> >
> > [1]: https://lore.kernel.org/linux-spi/20220512163445.6dcca126@erd992/
> >
> > > (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@xxxxxxxxxxxxxx/T/#u

I found one more. A patch from Martin with the basic proposed API but
not much in the way of implementation. It looks like this is where the
idea fizzled out.

https://lore.kernel.org/linux-spi/0C7D5B1E-E561-4F52-BEA8-572EB0CA26A6@xxxxxxxxxxxxxxxx/