Re: [PATCH RFC 5/9] ALSA: core: selection of audio_tstamp type and accuracy reports

[Takashi Iwai <tiwai@suse.de>]

At Thu, 11 Dec 2014 20:36:48 -0600,
Pierre-Louis Bossart wrote:
> 
> 
> >> if someone used alsa-lib with the .get_wall_clock(), the new user-space
> >> code will provide the same results as today, no change (wall clock if
> >> supported, hw_ptr otherwise). So the library compatibility is preserved.
> >
> > You can't assume that all users always upgrade alsa-lib.
> > Users may use still the old alsa-lib with the new kernel.
> >
> >> I don't mind adding a compatible kernel behavior for HDAudio only, but
> >> is this really necessary?
> >
> > Yes, the kernel is not allowed to give any regression, if we know it
> > would.
> 
> ok, will add a backwards-compatible mode. no problem.
> 
> >> I added a set of INFO defines and the matching is_supported queries in
> >> alsa-lib. I just did a pretty dumb copy/paste/edit there, maybe we can
> >> refactor the code here with a single routine taking a type parameter.
> >> feedback welcome there.
> >
> > Right.  But another concern is that this method will consume one INFO
> > bit at each time the new tstamp type is extended.  This is another
> > concern.  Exposing this information in another place would be better,
> > IMO, if any better place is found...
> 
> I asked about this one in late October... I mentioned that we are 
> running out of INFO (half already used) and AZX_CAPS (28 bits used) 
> fields, and for INFO it didn't seem like a big deal.

The INFO bits are the public ABI and can't be changed later while
AZX_DCAPS is the internal stuff we can change at any time.  So,
defining a new stuff for SND_PCM_INFO must be done very carefully.


> If adding more fields to the info field is viewed as problematic, the 
> only options I can think of are:
> - reclaim a reserved word in hw_params, e.g. rename to info1 to do 
> something like this in alsa-lib:
> 	return !!(params->info1 & SNDRV_PCM_INFO1_IS_THIS_HARDWARE_BROKEN)

It's OK to just add a new hw_param_mask element.  Then we can handle
up to 32 tstamp types and that should be enough.

The question is whether hw_params is the best place.  Usually
hw_params is the place to select the one configuration from multiple
choices or space.  In this case, we don't choose only one, right?

> - keep a 32 bit word but add a paging register in the msb to reuse lsbs.
> Either way some more code will be required in both driver and library.

Which word to reuse?  Could you elaborate?


thanks,

Takashi