Re: [PATCH 1/4] ALSA: control: return payload length for TLV operation

[Charles Keepax <ckeepax@opensource.wolfsonmicro.com>]

On Wed, Aug 31, 2016 at 03:24:31PM +0200, Clemens Ladisch wrote:
> Charles Keepax wrote:
> > On Wed, Aug 31, 2016 at 01:54:37PM +0200, Clemens Ladisch wrote:
> >> Anyway, the separate length value could be useful only for drivers (like
> >> soc-wm-adsp) that use a binary stream where TLV data should have been
> >> used.  But the software that writes these coefficients to the WM ADSP
> >> driver is very hardware specific anyway, and it presumably already works
> >> without knowing the returned length value.  So there is no case where
> >> this patchset _actually_ improves the interface.
> >
> > The software that writes these coefficients to wm_adsp is
> > not hardware specific at all its just regular amixer and
> > tinymix.
> 
> As far as I can see, neither amixer nor tinymix support writing
> or "command"ing TLV data.  Do you mean alsactl or alsaucm?
> 
> (And I notice that when alsa-lib's UCM loads TLV data from a file,
> it does check that the second word contains the correct size.  Is
> this value also correct when reading TLV from these controls?)
> 

Certainly tinyalsa does:

https://github.com/tinyalsa/tinyalsa/commit/45b2d047b8c2f4d9d1d87244f7f981db8234c906

I thought the Intel guys added support to amixer as well although
I may have been mistaken about that. I will try to find some time
to look at that a little more closely. I guess my main concern
here was the "very hardware specific" the intention is not to
require hardware specific user-space code to use these controls.

> > These TLV controls are just like any other ALSA control
> 
> You're treating it like one, but actually TLV is not a control type but
> metadata attached to a control.
> 

Yes but that metadata is just being used to transfer the actual
data for the control in this case. Personally I would just expect
that control to function the same way as any other binary control
for the user, just it is >512 bytes. Different things happen on
the back end but it would be nice if it felt the same to the
person using it. Which is mostly the case though tinyalsa at the
moment.

Thanks,
Charles