[alsa-devel] What does snd_pcm_delay() actually return?
tiwai at suse.de
Fri Jun 13 18:52:03 CEST 2008
At Fri, 13 Jun 2008 18:47:48 +0200 (CEST),
Jaroslav Kysela wrote:
> On Fri, 13 Jun 2008, Takashi Iwai wrote:
> > At Fri, 13 Jun 2008 18:11:12 +0200 (CEST),
> > Jaroslav Kysela wrote:
> > >
> > > On Fri, 13 Jun 2008, Takashi Iwai wrote:
> > >
> > > > What about just providing three pointers: curr_ptr, hw_ptr and
> > > > appl_ptr? curr_ptr corresponds to the point being played, and hw_ptr
> > > > is the point where the data was already sent to h/w, and appl_ptr is
> > > > the point where the data is filled by user. The above definitions are
> > > > all combinations of these pointers.
> > >
> > > But I think that curr_ptr can be managed in drivers, thus invisible to
> > > user space (except for snd_pcm_delay() propagation).
> > Ditto for hw_ptr. Why is it hidden at all?
> Does it improve something to show this pointer to apps? I don't see any
> reason to show it outside alsa-lib.
Then it'll be more clear.
> > > If driver requires
> > > extra handling of samples, it can allocate and manage extra buffers
> > > itself. I don't see the point to have "locked" samples already processed
> > > by hardware in the main ring buffer described by appl_ptr / hw_ptr.
> > > Application can use this space for new samples.
> > >
> > > The only advantage with your implementation might be zero-copy, but USB
> > > and PCMCIA cards have or create own buffers, so I don't think that this
> > > advantage can be used in actual drivers and I cannot even imagine
> > > hardware which work in way to use zero-copy in this situation.
> > Wait, wait. Please don't mix up. The above doesn't imply anything
> > about the further implementation of usb-audio driver. What I
> > suggested is, instead of hiding two pointers (hw_ptr and curr_ptr) and
> > creating a complex API, simply expose them.
> I don't see a reason to make current API more complex.
Because the current API is complex and hard to understand.
> We have already two
> functions, One showing overall latency and second one how much samples can
> be processed by application. It's enough. We need only improve things
> internaly in alsa-lib <-> kernel (provide correct information for
> > Now, regarding the usb-driver. Honestly, I don't understand what you
> > want to do with an extra URB.
> Note that we don't need to have extra URBs, just change hw_ptr handling
> in USB driver.
OK, then it's different from your previous explanation...
> > As now, usb-audio driver handles as curr_ptr == hw_ptr. But, in
> > reality, curr_ptr = hw_ptr - samples_in_urbs. So, in the case
> > of USB-audio, hw_ptr is ahead of curr_ptr. (And the granularity is
> > samples_in_urbs).
> As Lennart mentioned, in this case you can reach underrun at different
> position than expected (when URB cannot be filled). In my case, you'll
> reach underrun exactly at point when whole ring buffer is drained. So
> application can better estimate queueing and also it makes things more
Hm, could you elaborate how to do this more exactly? That wasn't
clear from your previous post at all.
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