[PATCH V3 1/2] of: Add generic device tree DMA helpers

[Jon Hunter]

On 06/14/2012 10:17 AM, Guennadi Liakhovetski wrote:
> Hi all
> 
> Sorry for jumping in so late in the game. But I think, the model, to which 
> this discussion is slowly converging, is not very well suitable for the 
> shdma DMACs, present on SH and ARM sh-mobile SoCs. I might be wrong and 
> the model is indeed suitable, in which case I'd be grateful, if someone 
> could explain, how this model could be used for shdma. Below I'll try to 
> describe main properties of shdma served DMACs, I've done this a couple of 
> times before during various dmaengine related discussions. Let's see how 
> we can use this model for these SoCs.
> 
> On Sat, 9 Jun 2012, Arnd Bergmann wrote:
> 
>> On Friday 08 June 2012, Jon Hunter wrote:
> 
> [snip]
> 
>>> It seems to me we were pretty close on alignment. In fact, I was quite
>>> happy with Steven's 2nd to last proposal of  ...
>>>
>>> simple 1 req:
>>> dmas = <0 &dmac1 xxx>;
>>>
>>> simple 2 req:
>>> dmas = <0 &dmac1 xxx 1 &dmac1 yyy>;
>>>
>>> multiple dmacs:
>>> dmas = <0 &dmac1 xxx 0 &dmac2 zzz 1 &dmac1 yyy>;
> 
> A typical sh-mobile SoC contains several DMAC instances of several 
> sub-types, all served by the same driver. Let's take sh7372 to be 
> specific (see arch/arm/mach-shmobile/setup-sh7372.c for details). On 
> sh7372 we've got 3 generic DMAC instances and 2 dedicated USB DMACs.
> 
> Generic DMACs have 6 physical channels each, USB DMACs 2.

For OMAP we also have dedicated DMAC for the USB controllers. However,
these DMAC are very much integrated into the USB controller itself.
Hence, in the of OMAP we would only be using the binding really to
handle the generic DMAC. However, I would not like to think that this is
limited to only generic DMAC.

> Generic DMACs can perform memory-to-memory DMA, USB DMACs cannot.
> 
> Generic DMACs can serve any slave (peripheral) request line on any their 
> physical channel, USB DMACs only serve fixed USB controller instances. To 
> configure (connect) a certain physical DMA channel to s specific 
> peripheral request line, a certain value has to be written to a 
> configuration register of that physical DMA channel.

Do you still need to specify a request line for the USB DMACs or are
these fixed? In other words, what purpose would the DMA binding serve
for the USB DMACs?

> To add to complexity, on other SoCs (e.g., SuperH sh7724) some of generic 
> DMACs (DMAC0) have external DMA request pins, others don't.

OMAP also has this. To me an request line going to an external pin can
be handled in the same way as one going to a internal peripheral.
However, what happens to that pin externally is a different story.

> I'm sure there's more to that, but that's about the scope, that's 
> currently supported or wants to be supported by the driver.
> 
> Currently we do the slave-switching in the following way: platform data 
> for each DMAC instance references a table of supported peripherals with 
> their IDs and configuration register values. Each peripheral, that wishes 
> to use DMA, provides its slave ID to the DMAC driver, by which it checks, 
> whether this peripheral is supported and, if so, finds its configuration, 
> picks up the next free channel and configures it.
> 
> So, with the above in mind, IIUC, the above DT binding proposal would 
> require us to reference all 3 generic DMAC instances in each slave dmas 
> property? 

You could if you wanted to have the ability to select 1 out of the 3.
However, I would not say it is a hard requirement. You could just
provide one. Or you could list all 3, but use some form of match
variable to indicate a default DMAC for a given peripheral.

> Even if we assume, that for this specific case we don't have to 
> map each physical channel, because so far they are "mostly" all equal on 
> each DMAC instance. Mostly, because external DMA request lines on sh7724 
> can only be used with channels 0 and 1 out of 6 of DMAC0... What we 
> certainly don't want to do is specify fixed physical DMA channels or even 
> controller instances in slave DMA bindings.

You could just use the binding to return a handle to one DMAC and then
just request a channel. I don't see that the binding would require you
to specify fixed channels.

> To me it looks like the above proposal would only very suboptimally be 
> usable with sh-mobile SoCs...

I guess I don't see that, but then again I am not familiar with the all
the details of the SH devices.

However, thinking about it some more, if a DMAC is this generic, what
information do you want the binding to provide?

Cheers
Jon