Subject: [PATCH] of: Add generic device tree DMA helpers

From: [email-masked] (Grant Likely)
Date: Sun, 18 Mar 2012 09:08:41 +0000

On Sat, 17 Mar 2012 16:03:02 +0000, Arnd Bergmann <[email-masked]> wrote:
> On Saturday 17 March 2012, Grant Likely wrote:
> > On Thu, 15 Mar 2012 09:26:52 +0000, Russell King - ARM Linux <[email-masked]> wrote:
> > > On Thu, Mar 15, 2012 at 09:22:06AM +0000, Arnd Bergmann wrote:
> > > > On Thursday 15 March 2012, Nicolas Ferre wrote:
> > > > > Add some basic helpers to retrieve a DMA controller device_node and the
> > > > > DMA request specifications. By making DMA controllers register a generic
> > > > > translation function, we allow the management of any type of DMA requests
> > > > > specification.
> > > > > The void * output of an of_dma_xlate() function that will be implemented
> > > > > by the DMA controller can carry any type of "dma-request" argument.
> > > > > 
> > > > > The DMA client will search its associated DMA controller in the list and
> > > > > call the registered of_dam_xlate() function to retrieve the request values.
> > > > > 
> > > > > One simple xlate function is provided for the "single number" type of
> > > > > request binding.
> > > > > 
> > > > > This implementation is independent from dmaengine so it can also be used
> > > > > by legacy drivers.
> > > > > 
> > > > > For legacy reason another API will export the DMA request number into a
> > > > > Linux resource of type IORESOURCE_DMA.
> > > > 
> > > > This looks very good. I missed the first version of this patch, but was
> > > > thinking of very similar bindings.
> > > 
> > > There's one issue which is concerning me - when we switch OMAP to use
> > > DMA engine, it won't use numeric stuff anymore but the DMA engine way
> > > of requesting a channel (match function + data).
> > > 
> > > How does that fit into this scheme?
> > 
> > Not well as the current patch set stands.  The xlate function doesn't return
> > any context for the dma channel number that it returns, so the driver cannot
> > figure out which DMA controller to use if there are multiple.
> 
> Shouldn't that be part of the data returned by xlate? I was under the impression
> that this would be the data that you would pass into dma_request_channel, together
> with a filter function that find the instance from the data.

Yes, that's my point.  The patch as it is currently written doesn't do
what it needs to.

g.