Re: [PATCH V6 1/2] of: Add generic device tree DMA helpers

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Saturday 15 September 2012, Russell King - ARM Linux wrote:
> 
> On Fri, Sep 14, 2012 at 05:41:56PM -0500, Jon Hunter wrote:
> > 3. Supporting legacy devices not using DMA Engine
> > 
> >    These devices present a problem, as there may not be a uniform way to easily
> >    support them with regard to device tree. Ideally, these should be migrated
> >    to DMA engine. However, if this is not possible, then they should still be
> >    able to use this binding, the only constaint imposed by this implementation
> >    is that when requesting a DMA channel via of_dma_request_slave_channel(), it
> >    will return a type of dma_chan.
> 
> As far as devices not using DMA engine, the answer is we don't support
> their specification in the DT model.  Note that the legacy OMAP DMA
> API is scheduled for removal next year, so it's not going to be around
> that much longer.

There are a few platforms using the ISA DMA API (rpc, h720x, shark, footbridge),
and I agree that they are unlikely to see OF support, although if they did, it
wouldn't be unreasonable to encode their DMA channels using the same binding.

The other ones that are currently around with their own DMA implementation are

bcmring --> platform is going away
samsung --> gradually getting moved to dmaengine, already has its own binding
            that needs to be replaced with this one, so best do it at the same
            time.
tegra   --> old dma code gone in 3.7
pxa/mmp --> dmaengine implementation being worked on, should wait for that.
msm     --> dma implementation only used by two drivers (serial and mmc). 

Outside of arch/arm, at least sh, cris, unicore32 and blackfin have their
own dma APIs based on the ISA interfaces. I don't currently see any of them
moving towards DT, but it's definitely possible. 

Among the above MSM seems to be the most likely candidate to use the binding
before moving to DT. The msm_sdcc driver is (like much of the msm platform
code) lagging far behind the internel version that qualcomm have, and the
device tree binding they are using is incompatible with the common MMC
binding (and of course the DMA binding here) as well. For getting MSM up
to speed compared with the other platforms, they have to use proper DT
bindings as well as proper DMA engine support. Between those two, I'd prefer
fixing the DT binding first, in order to limit the amount of changes that
have to be done to external device tree files.

	Arnd