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

From: Mason
Date: Fri Dec 09 2016 - 13:24:10 EST

[ Dropping Mans to preserve his peace-of-mind ]

On 09/12/2016 18:56, Vinod Koul wrote:
> On Fri, Dec 09, 2016 at 06:34:15PM +0100, Mason wrote:
>> 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.
> Right, but in that case the fallback would be PIO mode, and if that is
> not availble (IIRC some f your devices don't) then reject the usage with

Maybe I'm missing something, but I don't see how that would help.
Take the NAND Flash controller driver, for instance. PIO is not
an option, because the ECC engine is tied to DMA.

And failing with -EAGAIN doesn't help the busy looping situation.
The caller should be put on some kind of queue to wait for a
"channel ready" event.