Re: [PATCH RFC v2 2/8] spi: add basic support for SPI offloading

["Nuno Sá" <noname.nuno@gmail.com>]

On Fri, 2024-05-10 at 19:44 -0500, David Lechner wrote:
> SPI offloading is a feature that allows the SPI controller to perform
> complex transfers without CPU intervention. This is useful, e.g. for
> high-speed data acquisition.
> 
> This patch adds the basic infrastructure to support SPI offloading. It
> introduces new callbacks that are to be implemented by controllers with
> offload capabilities.
> 
> On SPI device probe, the standard spi-offloads devicetree property is
> parsed and passed to the controller driver to reserve the resources
> requested by the peripheral via the map_channel() callback.
> 
> The peripheral driver can then use spi_offload_prepare() to load a SPI
> message into the offload hardware.
> 
> If the controller supports it, this message can then be passed to the
> SPI message queue as if it was a normal message. Future patches will
> will also implement a way to use a hardware trigger to start the message
> transfers rather than going through the message queue.
> 
> Signed-off-by: David Lechner <dlechner@baylibre.com>
> ---
> 
> v2 changes:
> 
> This is a rework of "spi: add core support for controllers with offload
> capabilities" from v1.
> 
> The spi_offload_get() function that Nuno didn't like is gone. Instead,
> there is now a mapping callback that uses the new generic devicetree
> binding to request resources automatically when a SPI device is probed.
> 

Yeah, what I disliked the most was adding the platform devices from spi-engine
driver and then the complexity in the IIO trigger part of it. I also didn't like
(as you said) for the peripheral to have to explicitly request an offload but,
IIRC, Mark was actually ok with the spi_offload_get/put() mechanism so let's see
what he has to say. He's main point (I think) was for the controllers to have a
way to know which offload instance is busy or not (but I guess controllers can
keep track of that as well with this approach and using the enable/disable
callbacks or the prepare/unprepare).

THB, I do like this approach (and it is what I had in mind) and it's simple
enough while covering what we know about this feature atm.

- Nuno Sá