Re: [Alsa-devel] Re: [OT] ALSA userspace API complexity

John Rigg
2006-01-10T00:16:17Z

On Mon, Jan 09, 2006 at 03:21:21PM -0800, David Lang wrote:
> On Mon, 9 Jan 2006, John Rigg wrote:
>
> >On Mon, Jan 09, 2006 at 01:58:00PM -0800, David Lang wrote:
> >>On Mon, 9 Jan 2006, René Rebe wrote:
> >>>>>
> >>>>>Also, when the data is already available as single streams in a
> >>>>>user-space
> >>>>>multi track application, why should it be forced interleaved, when the
> >>>>>hardware
> >>>>>could handle the format just fine?
> >>>>Because the conversion doesn't cost anything. Trying to avoid it by
> >>>>making the API more complicated (I would even say confusing) is extreme
> >>>>overkill.
> >>>
> >>>Since when doesn't cost convesion anything? I'm able to count a lot of
> >>>wasted
> >>>CPU cycles in there ...
> >>
> >>if the data needed to be accessed by the CPU anyway it's free becouse
> >>otherwise the CPU would stall waiting for the next chunk of memory. you
> >>can do quite a bit of work on data in cache while you are waiting for the
> >>next cache line to load.
> >>
> >>in this same way, checksumming a network packet is free if the CPU needs
> >>to copy the data anway, it only costs something if the data could bypass
> >>the CPU.
> >
> >Yes, but the CPU has plenty of other work to do. The sound cards that
> >would be worst affected by this are the big RME cards (non-interleaved) and
> >multiple ice1712 cards (non-interleaved blocks of interleaved data),
> >which AFAIK are the only cards capable of handling serious professional
> >audio.
> >This could represent 48 or more channels of 96kHz audio, which
> >doesn't leave a lot of spare CPU capacity for running X, for example.
>
> does the CPU touch the data for these, or do you DMA directly from
> userspace (i.e. "zero-copy")?

The cards I mentioned use DMA. RME actually advertises that some of their
cards can handle 52 channels with zero CPU load. Their onboard DSPs can
also do routing and mixing, again without touching the CPU.

John