[PATCH] Documentation: dmaengine: Add a documentation for the dma controller API

[maxime.ripard@free-electrons.com (Maxime Ripard)]

Hi Lars-Peter,

On Thu, Jul 31, 2014 at 02:44:56PM +0200, Lars-Peter Clausen wrote:
> On 07/31/2014 09:44 AM, Maxime Ripard wrote:
> [...]
> >   - Having to set device_slave_caps or not?
> 
> Yes. This should in my opinion be mandatory for new drivers.

Ok. 

> One of the issues with the DMAengine API is that it is really hard
> to write generic drivers that do not already know about the
> capabilities of the DMA controller. E.g. if you have a peripheral
> that is used on SoC A it assumes that the DMA controller it is
> connected to has the capabilities of the DMA controller in SoC A. If
> the same peripheral is now used on SoC B with a DMA controller with
> different capabilities this often ends up with ugly ifdefery in the
> peripheral driver. The peripheral driver shouldn't have to know
> which specific DMA controller it is connected to (It's a bit as if a
> GPIO consumer needed to know which GPIO controller is connected
> to). We got away with the current approach since there is not that
> much diversity in the mixing of peripherals and DMA controllers
> (each vendor pretty has their own DMA controller which it uses for
> their own peripherals). But with more recent code consolidation we
> are on a path to generic DMA support within subsystem frameworks
> (there is generic DMA support for audio, there is generic DMA
> support for SPI and I also have a (currently) out of tree patch for
> generic DMA support for IIO). Also these generic drivers need to be
> able to discover the capabilities of the DMA controller to be able
> to make the right decisions.

Yeah, I've seen the generic infrastructure in both ASoC and SPI, and
it's great that it's coming to IIO as well.

I wasn't aware that it was relying on device_slave_caps though, and
been mislead by the caps name into thinking that it was related to the
caps_mask, which is obviously not.

>From what you're saying, and judging from the drivers that already
implement it, can't it be moved directly to the framework itself ?

The informations put there could be either used elsewhere (like
framework-level filtering of invalid directions/bus width) or could be
derived directly from which callbacks are set (in the pause/terminate
case)?

I guess that would make generic layer much easier to write, since
you'll always have this information.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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