[alsa-devel] ioctl(fd, SNDRV_CTL_IOCTL_ELEM_REPLACE, ... not working from userspace ?

[Takashi Sakamoto]

Hi,

On Jul 4 2017 01:50, Fulup Ar Foll wrote:
> At kernel level the iotcl point onto ./sound/core/control.c and calls
> snd_ctl_elem_add_user(ctl, argp, 1) that itself calls snd_ctl_elem_add
> 
> Looking at this code I do not understand why the replace iotcl is
> refused. Nevertheless it seems more to come from some lacking data in ly
> ctrlinfo than because of access right (I'm testing as root)

ALSA control core in kernel land is designed to maintain two types of
control element set; one is added by drivers in kernel land, another is
added by userspace applications. The macro, 'SNDRV_CTL_ELEM_ACCESS_USER'
is for distinction of the latter type from the former type. Both type of
control element set can be handled by userspace applications in the same
way (read/write/poll/ioctl with SNDRV_PCM_IOCTL_XXX), however they have
different effects.

Typically, users utilize the former type of control element set to
configure hardware features with helps of kernel drivers. On the other
hand, the latter type of control element set has no direct relations to
hardware. There's a need for userspace applications to handle any events
relevant to the latter type of control element set when achieving some
actual effects. At present, there's a few cases to use the latter type;
e.g. softvol PCM plugin in alsa-lib utilizes the latter control element
set to add alternative control element set for PCM frame resampling[1].

The issued operation, SNDRV_CTL_IOCTL_ELEM_REPLACE, is designed just for
the latter type of control element set. Therefore, you got no success
from this operation for the former type of control element set. When
attempting to get success, you need to add the latter type of control
element set in advance, then issue the operation for the added control
element set. You can see a test program in alsa-lib[2] for the latter
type of control element set, for your information.

But I know that implementations for the latter type of control element
set has been long-abandoned. For recent years I had been fixing them but
some bugs might be still left, perhaps. If you still receive some errors
from this operation for the latter type of control element set, please
report how to reproduce it (some programs are preferable). I'm willing
to investigate and fix the bug.

[1] src/pcm/pcm_softvol.c
http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/pcm/pcm_softvol.c
[2] test/user-ctl-element-set.c
http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=test/user-ctl-element-set.c


Regards

Takashi Sakamoto