[RFC 03/34] dmaengine: virt-dma: add support for cyclic DMA periodic callbacks

[zhangfei gao]

On Fri, Jun 1, 2012 at 3:17 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Thu, May 31, 2012 at 11:45:54AM +0800, zhangfei gao wrote:
>> On Wed, May 30, 2012 at 4:02 PM, Russell King - ARM Linux
>> <linux at arm.linux.org.uk> wrote:
>> > 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.