Re: [PATCH V2] dma: tegra: register as an OF DMA controller

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

On 12/03/2013 06:22 PM, Arnd Bergmann wrote:
> On Tuesday 03 December 2013, Stephen Warren wrote:
>> On 11/29/2013 02:08 PM, Arnd Bergmann wrote:
> 
>>> Can you try coming up with a different method to achieve the same
>>> where you use a different helper from the driver specific xlate
>>> function that does not require a callback?
>>>
>>> I think dma_get_slave_channel is great if you have one channel per
>>> request line and you can directly look up the channel from the
>>> DT data, but it is not good if you have pick a channel and work
>>> around the race.
>>
>> Hmm. Can you take a look at "[PATCH V4] dma: add
>> dma_get_any_slave_channel(), for use in of_xlate()" at the link below.
>> It still implements this via xlate, but I don't see any benefit in
>> making drivers use a different API to request slave channels based on
>> how the DMA controller works.
>>
>> http://lkml.org/lkml/2013/11/26/408
> 
> Yes, I think that is good. I can think of a few variations of that
> that I would prefer slightly over your code, but it's essentially
> what I had in mind and I'm fine with that version getting merged
> as well. Here are my ideas for further improvements, I'll leave
> it up to you and the dmaengine maintainers to decide what to do
> about them:
> 
> * Rather than calling private_candidate(), open-code the part you
>   need and remove the pointless dma_cap_mask comparison:
> 
> 	err = -EBUSY;
>         list_for_each_entry(chan, &dev->channels, device_node) {
>                 if (!chan->client_count) {
> 			err = dma_chan_get(chan);
> 			break;
> 		}
> 	}

Lars-Peter had specifically suggested to call private_candidate(). Lars,
what do you think about open-coding this? Arnd's suggestion would skip
the DMA_PRIVATE checking that private_candidate() does, and I'm not sure
what the implications of that would be.

> * Merge the new function with dma_get_slave_channel(). They really
>   do different things, but I think it still makes sense as an API
>   to require to always pass the dma_device pointer, and drivers
>   that want to get an arbitrary channel can just pass NULL as the
>   channel pointer.

I suppose one could do that, although the two operations seem pretty
semantically different to me, such that merging them doesn't seem correct.