[alsa-devel] [RFC] ALSA: usb-audio: Add custom mixer status quirks for RME CC devices

[Jussi Laako]

On 21.09.2018 11:43, Takashi Iwai wrote:
> On Fri, 21 Sep 2018 10:35:49 +0200,
> Jussi Laako wrote:
>>
>>> The caching is currently enabled for all elements, but changing it
>>> should be trivial.  The patch below adds is_volatile flag to the
>>> element, and you can set it to true in the quirk somehow for uncached
>>> controls.
>>
>> This patch looks good and what I need! When trying to figure out where
>> it is safe to set the volatile flag I thought that I could set it
>> transparently in add_single_ctl_with_resume() in mixer_quirks.c based
>> on the SNDRV_CTL_ELEM_ACCESS_VOLATILE flag in provided
>> snd_kcontrol_new access-member. However, add_single_ctl_with_resume()
>> is allocating just size of usb_mixer_elem_list unlike
>> snd_create_std_mono_ctl_offset() which in turn is allocating full
>> usb_mixer_elem_info size. However, the mixer code seems to be assuming
>> that the item is always usb_mixer_elem_info instead of just list
>> header item. Is this allocation behavior correct and is the item
>> turned into usb_mixer_elem_info somewhere, or is this some kind of
>> bug? So can I safely turn the allocation in
>> add_single_ctl_with_resume() into zero initialized usb_mixer_elem_info
>> instead and set the flag there while keeping correct behavior, or am I
>> missing something?
> 
> We can check SNDRV_CTL_ELEM_ACCESS_VOLATILE instead of introducing a
> new flag like my patch, yes.  It's just a matter of easiness.
> Since there is no link from usb_mixer_elem_info to the assigned
> snd_kcontorl, we'd need to add an extra argument in the call path to
> inform about the volatileness, and it may become messy.  We'll find
> the simplest way.
> 
> And, IMO, it'd be better to check the cache behavior at setting, since
> the cache may be accessed via multiple paths (at reading each element
> and at resuming the whole system).  But it's no requirement, either.

I have tried with attached patch, on top of your patch, but I'm still 
getting cached values from libasound2. It didn't break anything 
either... However, I don't see a link between this cache behavior and 
specific user space mixer descriptor. Because polling the mixer element 
with snd_mixer_selem_get_capture_volume() keeps always returning the 
same value. While in parallel checking the value by running "amixer" 
from command line gives updated correct value... So is there a second 
layer of cache somewhere in libasound2 to explain this discrepancy? 
Because one process sees updated value while other one doesn't. 
Difference being that the ALSA descriptors are closed and reopened every 
time "amixer" is run...

What is more perplexing is that from HDSPe driver same use case works, 
but it could be due to some event trigger from the HDSPe driver based on 
interrupt?


	- Jussi
-------------- next part --------------
A non-text attachment was scrubbed...
Name: tmp.patch
Type: text/x-patch
Size: 973 bytes
Desc: not available
URL: <http://mailman.alsa-project.org/pipermail/alsa-devel/attachments/20180921/6f1d81ee/attachment.bin>