Re: New PCM API extension

[Abramo Bagnara <abramo.bagnara@libero.it>]

Jaroslav Kysela wrote:
> 
> I think that I have one present problem. It was not possible to write,
> something like the dmix plugin does (in the server/client model): Transfer
> data to different areas in the ring buffer. Imagine that client1 wants to
> write data at the end of the ring buffer (assume that server don't change
> the r/w pointer). Then client2 comes with some data to be placed at the
> start of the buffer later (server rewinds the stream position). Then
> hardware processed a period and client1 wants to continue (server forwards
> the stream position at the end of the ring buffer).
> 
> I can imagine more examples where "later" samples might be wanted to be
> processed before "quick" ones.

Your examples appears a little confused at my eyes, but if I've
understood correctly don't you think this might be solved by
snd_pcm_rewind done by server?

At least this is how it works in pcm_share.

> > > I'm also going to propose next extension: Actually, the appl_ptr managing
> > > routines - snd_pcm_reset(), snd_pcm_rewind(), snd_pcm_forward() always
> > > uses an ioctl for it's operation. I think that it wouldn't be a bad idea
> > > to implement these function in the lightweight variant, too. I mean that
> > > they will operate only with the actual hw_ptr without calling the kernel
> > > code. We have already functions to synchronize the hw_ptr with the
> > > hardware (snd_pcm_delay(), snd_pcm_hwsync()). Because it's late to change
> > > this behaviour in alsa-lib, I propose to create a function
> > > 'snd_pcm_fast_rw_change()' (better name?) which tells the alsa-lib that no
> > > accurate operation is required. Especially lowlatency applications will
> > > benefit that we removed more 'user<->kernel' switches.
> >
> > Note that the assumption of monotonic direction of appl_ptr is spread
> > everywhere so I doubt that this is feasible for snd_pcm_rewind.
> 
> I don't understand here. I've looked to the ioctl implementation and there
> are no problems to change appl_ptr in the user space only as well.

Have you thought what happens if appl_ptr is changed backward during
interrupt handler?

Lock free ring buffer absolute precondition is monotonic direction.

-- 
Abramo Bagnara                       mailto:abramo.bagnara@libero.it

Opera Unica                          Phone: +39.546.656023
Via Emilia Interna, 140
48014 Castel Bolognese (RA) - Italy


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