Re: dmaengine: query current transfer status

From: Guennadi Liakhovetski
Date: Tue Jan 12 2010 - 03:02:30 EST

On Mon, 11 Jan 2010, Dan Williams wrote:

> On Mon, Jan 11, 2010 at 7:59 AM, Guennadi Liakhovetski
> <g.liakhovetski@xxxxxx> wrote:
> > Hi
> >
> > I'm using the dmaengine API with an ASoC driver (soon to be submitted),
> > and I have to provide a callback to ALSA to query the progress of the
> > current transfer (the .pointer() method from struct snd_pcm_ops).
> > Currently I only can provide this information with a period size
> > precision. Wouldn't it make sense to add an optional hook to dmaengine for
> > that? I certainly could add this as a platform-specific extension to the
> > specific dmaengine driver, I'm using (shdma), but that'd be ugly. Ideas?
> > Specific proposals for the prototype? I could cook up a patch if there are
> > no principal objections against this.
> No principal objections, but we already have a method for querying the
> status of a transaction ->device_is_tx_complete(). I'd rather change
> the semantics of this method to return the inflight state of the
> operation in addition to the channel's progress.

Yes, I was thinking about reusing it too, slthough, this would require
modifying all existing dmaengine implementations and users of
->device_is_tx_complete(), dma_async_is_tx_complete(), and
dma_async_memcpy_complete() methods. Not implssible but a bit cumbersome
in a single patch.

> Perhaps pass in a
> pointer to a new dma_tx_info (name?) struct

Yeah, I would suggest struct dma_tx_status, but that could be confusing
with enum dma_status.

> that gets filled in with
> the cookie information in addition to the current position of the
> operation in question.
> Something like:
> struct dma_tx_info {
> dma_cookie_t last;
> dma_cookie_t used;
> unsigned long pos;


> }
> enum dma_status (*device_is_tx_complete)(struct dma_chan *chan,
> dma_cookie_t cookie, struct dma_tx_info *info)

Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
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