Re: [PATCH v3 2/5] dmaengine: Add STM32 DMAMUX driver

[Peter Ujfalusi]

On 2017-08-02 07:55, Vinod Koul wrote:
> On Tue, Aug 01, 2017 at 09:32:50AM +0000, Pierre Yves MORDRET wrote:
> > On 07/31/2017 02:31 PM, Vinod Koul wrote:
> > > On Wed, Jul 26, 2017 at 07:38:02AM +0000, Pierre Yves MORDRET wrote:
> > > > > > > > +
> > > > > > > > +#ifndef __DMA_STM32_DMAMUX_H
> > > > > > > > +#define __DMA_STM32_DMAMUX_H
> > > > > > > > +
> > > > > > > > +#if defined(CONFIG_STM32_DMAMUX)
> > > > > > > > +int stm32_dmamux_set_config(struct device *dev, void *route_data, u32 chan_id);
> > > > > > >
> > > > > > > Why do we need a custom API in this case?
> > > > > >
> > > > > > This API is called by DMA when a slave is requested by client. DMA can work
> > > > > > without DMAMUX this API has been put in place to configure DMAMUX whether client
> > > > > > is requesting a DMAMUX Channel instead of a DMA one.
> > > > >
> > > > > You mean the dmaengine driver right?
> > > >
> > > > Yes. The API is mainly called by "device_config" through out STM32 DMA Driver
> > > > when a router is in place for client.
> > > > Please refer to Patch 4/5 on this set.
> > >
> > > Okay am thinking on why this can't be generic..? An optional router config
> > > callback?
> >
> > I would have liked to answer there is a callback within engine but unfortunately
> > I didn't figure out one when I did my router's development. I've looked once
> > more but again I can't find how to map chanID and request line without custom API.
>
> Yes there is no callback for routers but we can add a generic callback
> here to be used. I added Peter for his comments, isn't that something they
> need too?

The event router via of_dma_router_register() should be capable of 
handling different type of muxers, like the ti-dma-crossbar.c is doing 
for two different type of event crossbars.

Basically with the of_dma_route_allocate you craft a dma_spec which can 
be understood by the dma-master pointed form the router's node.
You do the configuration of the mux in this function, craft the dma_spec 
and that's it. In DT the peripherals are using the router's node for DMA 
binding and everything is transparent for them.

Note: The use of am335x xbar in the ti-dma-crossbar is optional and only 
needed when we need to have different event than the default for a 
specific dma request line.

If you normally use the DMA like this:
dmas = <&edma 129 1>, <&ddma_xbar 128 1>;
dma-names = "tx", "rx";

If you have DMA event router/mux, then depending on how it works you 
might have different number of parameters. In my case the DRA7 crossbar 
does not need extra parameter, so to get it in use:
dmas = <&edma_xbar 129 1>, <&edma_xbar 128 1>;
dma-names = "tx", "rx";

The router driver will rewrite the dma_spec and replace the 129/128 and 
pass something different to the dma-master (dynamic event mapping).

On am335x we have different xbar type so there:

dmas = <&edma_xbar 12 0 1>, <&edma_xbar 13 0 2>;

Out from this the router driver will create a spec equivalent to
dmas = <&edma_xbar 12 0>, <&edma_xbar 13 0>;

But it will change the xbar that DMA request 12/13 will not have the 
default event routed to.

I believe that the dma_router infra we have in dmaengine can cover most 
if not all use cases...

- Péter