Re: Tearing down DMA transfer setup after DMA client has finished

From: Mason
Date: Fri Dec 09 2016 - 12:35:09 EST

On 09/12/2016 18:17, Vinod Koul wrote:

> On Fri, Dec 09, 2016 at 11:25:57AM +0100, Sebastian Frias wrote:
>> What concrete solution do you propose?
> I have already proposed two solutions.
> A) Request a channel only when you need it. Obviously we can't do virtual
> channels with this (though we should still use virt-channels framework).
> The sbox setup and teardown can be done as part of channel request and
> freeup. PL08x already does this.
> Downside is that we can only have as many consumers at a time as channels.
> I have not heard any technical reason for not doing this apart from drivers
> grab the channel at probe, which is incorrect and needs to be fixed
> irrespective of the problem at hand.
> This is my preferred option.

There is one important drawback with this solution. If a driver calls
dma_request_chan() when no channels are currently available, it will
get -EBUSY. If there were a flag in dma_request_chan to be put to
sleep (with timeout) until a channel is available, then it would
work. But busy waiting in the client driver is a waste of power.