[alsa-devel] [PATCH] ALSA: ctl: allow TLV read operation for callback type of element in locked case

[Takashi Sakamoto]

On Mon, Dec 23, 2019 at 06:33:47PM +0900, Takashi Sakamoto wrote:
> At present, when an element has callback function for TLV information,
> TLV read operation returns EPERM if the element is locked. On the
> other hand, the read operation is success when an element has allocated
> array for TLV information. In both cases, read operation is success for
> element value expectedly.

You can regenerate the issue by executing below Python 3 scripts:
(Installation of alsa-gobject[1], PyGObject[2] and amixer(1) in
alsa-utils is required.)

After element is locked, amixer reports EPERM for any read operation of
TLV information.

```
#########after locked#########
numid=28,iface=PCM,name='Playback Channel Map'
  ; type=INTEGER,access=r---lR--,values=6,min=0,max=36,step=0
  : values=0,0,0,0,0,0
amixer: Control hw:1 element TLV read error: Operation not permitted
```

======== 8< --------
#!/usr/bin/env python3

from sys import argv,exit
import subprocess

import gi
gi.require_version('ALSACtl', '0.0')
from gi.repository import ALSACtl

def lock_elems(card, targets, locked):
    for target in targets:
        card.lock_elem(target, locked)


def run_amixer(label, targets):
    print('{:#^30}'.format(label))
    args = ['amixer', '-c', str(card_id), 'cget', '(placeholder)']
    for target in targets:
        args[4] = "iface=PCM,name='{}'".format(target.get_name())
        subprocess.run(args)


if len(argv) < 2:
    print('One argument is required for the numerical ID of soundcard.')
    exit(1)
card_id = int(argv[1])

card = ALSACtl.Card.new()
card.open(card_id)

targets = list(filter(lambda e: e.get_name().find("Channel Map") >= 0,
                  card.get_elem_id_list()))
run_amixer('before locked', targets)

lock_elems(card, targets, True)
run_amixer('after locked', targets)

lock_elems(card, targets, False)
run_amixer('after released', targets)

# The lock is surely released automatically when control character device is
# released. Thus it's safe to terminate the above codes in its middle.
======== 8< --------

[1] https://github.com/alsa-project/alsa-gobject/
[2] https://pygobject.readthedocs.io/en/latest/