On Fri, Jun 1, 2012 at 3:17 AM, Russell King - ARM Linux
[off-list ref] wrote:

On Thu, May 31, 2012 at 11:45:54AM +0800, zhangfei gao wrote:

quoted

On Wed, May 30, 2012 at 4:02 PM, Russell King - ARM Linux
[off-list ref] wrote:

quoted

On Wed, May 30, 2012 at 03:52:53PM +0800, Linus Walleij wrote:
No, I don't think we should do that - ALSA doesn't require it, and ALSA
is about the only user of this. ?As I've said, DMA engine drivers should
implement a correct tx_status function, which returns the remaining bytes
to be transferred to the end of the buffer so it's possible to find out
how many periods have completed when the callback is called.

Could we directly use pos, but unmatch with dma_tx_state.residue.
For cyclic buffer, when pos reach to buffer size, pos will change to 0,
remaining bytes also 0, we may have to distinguish them.

What pos?

The way I said above, you work out how many periods have expired by
using dma_tx_state.residue. ?This tells you how many bytes are left to
transfer until the _end_ of the looped buffer.

From that, you can tell directly how many periods have expired.

Really, ALSA is not interested in "how many periods have expired" but
is really after a regular update of where DMA is _now_, so it knows
how many bytes it can write into or read from the circular buffer. ?It
really doesn't need to know that N periods have expired _if_ you're
capable of telling it exactly where the DMA is in the buffer.

And with a working dma_tx_state.residue, you can do exactly that.

Thanks a lot.
Could we directly treat dma_tx_state.residue as "how many periods have expired"
or exactly where the DMA is in the buffer.
But the @residue: the remaining number of bytes left.
So if we set residue as remaining bytes,
dma driver: set residue = buf_len - expired bytes (position).
alsa driver: get expired bytes (position) = buf_len - residue.
But it is not a big deal.