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

[Jaroslav Kysela]

Dne 20. 01. 21 v 10:27 Takashi Iwai napsal(a):
> 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.

I agree. The UINT_MAX limit is really big in my eyes. The condition was added
to check for the insane allocations. I basically agree to increase this limit
(512, 1024?), but it should not be UINT_MAX.

						Jaroslav

-- 
Jaroslav Kysela <perex at perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.