[PATCH] ALSA: control: remove limitation on the number of user-defined control element set per card

[Takashi Iwai]

On Wed, 20 Jan 2021 09:55:41 +0100,
Takashi Sakamoto wrote:
> 
> ALSA control core allows usespace application to register control element
> set by call of ioctl(2) with SNDRV_CTL_IOCTL_ELEM_ADD request. The added
> control element set is called as 'user-defined'. Currently sound card has
> limitation on the number of the user-defined control element set up
> to 32.
> 
> The limitation is inconvenient to drivers in ALSA firewire stack since
> the drivers expect userspace applications to implement function to
> control device functionalities such as mixing and routing. As the
> userspace application, snd-firewire-ctl-services project starts:
> https://github.com/alsa-project/snd-firewire-ctl-services/
> 
> The project supports many devices supported by ALSA firewire stack. The
> limitation is mostly good since routing and mixing controls can be
> represented by control element set, which includes control element with
> the same parameters. Nevertheless, it's actually inconvenient to device
> which has many varied functionalities. For example, plugin effect such as
> channel strip and reverb has many parameters. For the case, many control
> elements are required to configure the parameters and control element set
> cannot aggregates controls for the parameters. At present, below models
> are implemented with the control elements and actually add control element
> sets over 32:
> 
>  * Apogee Emsemble (snd-bebob-ctl-service)
>  * TC Electronic Konnekt 24d (snd-dice-ctl-service)
>  * TC Electronic Studio Konnekt 48 (snd-dice-ctl-service)
>  * TC Electronic Konnekt Live (snd-dice-ctl-service)
>  * TC Electronic Impact Twin (snd-dice-ctl-service)
> 
> It could be investigated to increase the number; e.g. quadruple to the
> current (=128), however it's hard to find criteria about the number for
> existent sound card. This commit just removes the limitation for the
> reason. ALSA control core uses UINT_MAX as the maximum number of control
> elements added to a sound card. It's limitation for both in-kernel driver
> and userspace application.
> 
> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>

I'm still worried by the unlimited number of possible additions.
Did you check what would happen if you run a test program to add
user-space ctls (with the max count) in a loop repeatedly?  If that
doesn't blow up too much, it might be OK.  Otherwise we have to add
some practical limits.

So, let's prove that it's absolutely safe to release the limit at
first.


thanks,

Takashi