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

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Date:   Wed, 2 Aug 2017 12:19:06 +0300
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     Vinod Koul <vinod.koul@...el.com>,
        Pierre Yves MORDRET <pierre-yves.mordret@...com>
CC:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre TORGUE <alexandre.torgue@...com>,
        Russell King <linux@...linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        "M'boumba Cedric Madianga" <cedric.madianga@...il.com>,
        Fabrice GASNIER <fabrice.gasnier@...com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Fabien DESSENNE <fabien.dessenne@...com>,
        Amelie DELAUNAY <amelie.delaunay@...com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/5] dmaengine: Add STM32 DMAMUX driver

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