[alsa-devel] UCM extensions

[Takashi Iwai]

On Thu, 07 Nov 2019 09:33:27 +0100,
Jaroslav Kysela wrote:
> 
> Dne 07. 11. 19 v 7:48 Takashi Iwai napsal(a):
> > On Tue, 05 Nov 2019 20:36:28 +0100,
> > Jaroslav Kysela wrote:
> >>
> >> Hi all,
> >>
> >> 	I make some internal ucm code cleanups in alsa-lib and added
> >> three major extensions to allow more complex configurations which we
> >> require for the SOF kernel driver.
> >>
> >> 	The first thing is the added substitution for the value strings:
> >>
> >> https://github.com/alsa-project/alsa-lib/commit/f1e637b285e8e04e6761248a070f58f3a8fde6fc
> >>
> >> 	The second thing is the If block:
> >>
> >> https://github.com/alsa-project/alsa-lib/commit/985715ce8148dc7ef62c8e3d8ce5a0c2ac51f8df
> >>
> >> 	The third thing is the card / hardware like specifier passed
> >> as the ucm name to snd_use_case_mgr_open() to support multiple card
> >> instances:
> >>
> >> https://github.com/alsa-project/alsa-lib/commit/60164fc5886cdc6ca55eeed0c2e3f751a7d2b2c0
> >>
> >> 	All those patches (with other cleanups) are in the ucm2 branch
> >> on github for comments:
> >>
> >> https://github.com/alsa-project/alsa-lib/commits/ucm2
> >>
> >> 	The proposed SOF UCM config diff is here:
> >>
> >> https://github.com/alsa-project/alsa-ucm-conf/commit/723b6da881721488229154e923ed36413955a051
> >> https://github.com/alsa-project/alsa-ucm-conf/commits/ucm2
> >>
> >> 	I added everything to keep the interface backward compatible,
> >> so the current applications should not observe any different
> >> behavior. The applications like pulseaudio should use the
> >> 'hw:CARD_INDEX' specifier for the open call in the future and
> >> snd_use_case_parse_ctl_elem_id() helper for the element control names.
> >
> > The only concern with these extensions so far is the compatibility.
> > Imagine that people run the new profile on the old parser, it'd break
> > easily.
> >
> > I think other scripts often installing on the versioned directory if
> > incompatibilities are seen.  Can we do that for UCM as well?
> >
> > Or course, once after UCM parser is changed to be future-ready and
> > allow some syntax for possible future extensions, we can keep that
> > version directory in future, too.
> 
> While we are going to separate UCM files from alsa-lib to
> alsa-ucm-conf we can define the new syntax change until the first
> version is released (I can put a notice to README).
> 
> Speaking for Fedora, we have dependancy 'alsa-lib package version'
> must be equal to 'alsa-ucm package version'. If users will do any
> changes on their own, they should know what they are doing.

This assumes that you have only one alsa-ucm package.  If there is a
downstream version of UCM profile, this won't work well always.

> Anyway, we should learn from this and I would propose to add add
> something like 'Syntax 2' to the main configuration file now. The new
> functions should be activated only according the version.

Yeah, some extensibility is needed in the config space.

> Unfortunately, the current parser will just show an error message like:
> 
> ALSA lib parser.c:1337:(parse_master_file) uknown master file field Syntax
> ALSA lib parser.c:1337:(parse_master_file) uknown master file field If

Right, that's the problem now.  Even a non-existing control would lead
to an error with the current version of parser.

> But at least, users should be notified that there is a configuration mismatch.

I don't think this would suffice.  The new UCM config is not merely a
config but it's becoming rather a language, so this needs some
distinction from the current v1 files.

> Another possibility is to change the suffix for the master
> configuration file to accept the "Syntax" check for the another future
> update. But honestly, I don't like ".conf2" and ".v2.conf" looks not
> so nice, too.

My vote is for a different directory.  And, with v2 extension, we
should leave the room for further extensibility, and keep the same
location as long as it's compatible.  Keeping the location for
incompatible configs would lead to a mess.


thanks,

Takashi