[alsa-devel] [RFC] virtual master control for HD-audio
tiwai at suse.de
Thu Jan 10 12:49:52 CET 2008
At Thu, 10 Jan 2008 11:29:39 +0100 (CET),
Jaroslav Kysela wrote:
> On Sat, 22 Dec 2007, Takashi Iwai wrote:
> > At Fri, 21 Dec 2007 20:37:09 +0100 (CET),
> > Jaroslav Kysela wrote:
> > >
> > > On Thu, 20 Dec 2007, Takashi Iwai wrote:
> > >
> > > > Hi,
> > > >
> > > > another thing I'd like to push into the next kernel is the virtual
> > > > master volume control. As I posted in earlier posts, it adds virtual
> > > > controls that have several slave controls with the same types,
> > > > e.g. Front, Surround, Center, LFE, etc. Then these are adjusted
> > > > simultaneously via the master control.
> > > >
> > > > It'd be appreciated if some one can test the patches with HD-audio h/w
> > > > that has no master control yet (e.g. some STAC codecs).
> > > >
> > > > Note that this won't add the master volumes if there are no real
> > > > volume controls. Some codecs have really no volume control, and this
> > > > won't help for such devices.
> > > >
> > > > Two (and one for driver) patches will follow after this.
> > >
> > > NAK from my side. I am convinced that this code can be implemented in the
> > > user space even without any daemon just in the mixer abstract layer using
> > > standard control elements and using eventually new user controls to store
> > > data for virtual mixer controls.
> > A user-space implementation of virtual mixer elements would be far
> > more complicated than the simplistic kernel-space patch. I've
> > considered it many times, even tried to implement it, but got that
> > conclusion. You'll see obviously the following difficulties:
> > 1. Many user-space virtual elements
> > Each slave control element needs a virtual element (eventually a
> > user-space one) because we need to keep both raw and virtual values to
> > handle saturation. That is, the same number of additional controls
> > would be added. Significant for 7.1 outputs.
> Note that these "extra" values can be handled together using only one
> user-space control element without "Volume/Switch" in name.
This method is very likely fragile. Think what happens if the number
of elements changes, or if the element numid changes. Then the packed
values will be screwed up.
> > 2. Easy incosistency between virtual and raw elements
> > Even if the mixer abstraction hides the virtual elements, both
> > virtual and raw elements are exposed on control API. This can cause
> > the value-inconsistency between them quite easily, because many apps
> > access directly via control API (even some mixer apps), and they
> > likely change only raw values. The similar situation is for kernel
> > OSS emulation.
> I have basically two ideas to handle this.
> 1) create a plugin system directly in the snd_ctl_* interface
> - basically code might be very same code as you proposed in kernel
No. The big difference is that the raw values are still exposed via
control API (otherwise how do you read/write raw values?). That is,
any user-space program may break consistency. OTOH, the kernel
solution doesn't expose raw values.
> 2) create virtual things only in smixer
> - it's something I strongly prefer
> - if mixers uses CTL interface, it's not our bussiness because
> we haven't claimed to create virtual layers in CTL
Hm, I have to say it's a bad attitude. It's our business to keep the
whole system consistent. We shouldn't build such a weak and fragile
system any more.
And what about the kernel OSS mixer? There are still many apps to
> > 3. Complicated configuration
> > The requirement of virtual master controls is very much dependent on
> > the hardware. In the case of HD-audio, it depends on codec chip
> > types, and even on the preset model chosen via PCI SSID or a module
> > option. Implementing such a complex conditional in alsa-lib
> > configuration is a clear overhead.
> I don't agree much here. Of course, we might need some hints from the
> driver (using information about components in sndrv_ctl_card_info) or we
> might analyze available controls using their names if Master control is
> present or we can eventually create a configuration tool saving hints to
> alsa-lib's configuration files which controls should be added for given
> hardware (this configuration tool might create a list of useable PCM
> devices handled in the driver, too). Just idea.
But it *is* an overhead.
> > 4. More resource requirement
> > Clearly a user-space solution requires more layers in alsa-lib, which
> > is invoked by each process, and even more memory footprint than the
> > kernel solution, not only the additional complexity.
> > ... and think again what is the benifit of the user-space solution in
> > this case, in comparison with the demerits above.
> The benefit is flexibility for future extensions.
The demerit is complexity even longing for future :)
And the worst thing is that it is not implemented yet *at all*. How
many years have we told that this kind of thing must be implemented in
Seriously, we need a solution RIGHT NOW.
> > If you could implement the same feature on user-space with less amount
> > of codes, I'd happily take it. But, I'm sure it's very hard.
> The question is also how to behave in future and if we should allow to
> break our basic rule in the current ALSA design - everything that can be
> moved to user space will be moved to user space.
Hey, it's just the question of balance. Insisting on a rule makes you
blind. The rule exists not for following rules. The rule exists for
creating a better system.
Note taht this isn't for solving every mixer problem. It doesn't
solve the world hunger. But, it solves nicely the really annoying
problem we face right now, without messy, fragile and overly complex
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