[alsa-devel] How ALSA handle PCM playing when the ALSA modules are removing
Hi, Takashi
I am doing an ALSA module test with insert module/remove module loop, when playing a wave during this loop, there will be kernel oops. Does ALSA need handle this case? Or just forbid wave playing during the module load/unload?
If this need to be handled, module unload will wait the wave play ending, or just stop the wave play at the first of the load/unload?
BR
Yingjiang
Hi,
(I'm another Takashi.)
On 2019/01/23 14:01, Zhu, Yingjiang wrote:
I am doing an ALSA module test with insert module/remove module loop, when playing a wave during this loop, there will be kernel oops.
It's just a bug of your modules.
Linux kernel module is maintained by reference counter. In Linux sound subsystem, when applications opens any character devices added by driver module, the reference counter is incremented, then it cannot be unload until all of character devices added by the module are released by applications.
If you can unload your module during transmission of audio data frame, it's apparently a bug of your module.
Does ALSA need handle this case? Or just forbid wave playing during the module load/unload? If this need to be handled, module unload will wait the wave play ending, or just stop the wave play at the first of the load/unload?
All of character devices for a sound card are associated to sound card structure (struct snd_card). Kernel modules should set 'disconnected' state to the structure in its callback for .remove invoked in unloading/unbinding operation. Then appliations receives errors in I/O operations to the character device. The applications are expected to close the character device. When all of character devices are closed, then sound card structure is going to be released.
Regards
Takashi Sakamoto
On 1/23/2019 1:22 PM, Takashi Sakamoto wrote:
Hi,
(I'm another Takashi.)
On 2019/01/23 14:01, Zhu, Yingjiang wrote:
I am doing an ALSA module test with insert module/remove module loop, when playing a wave during this loop, there will be kernel oops.
It's just a bug of your modules.
Linux kernel module is maintained by reference counter. In Linux sound subsystem, when applications opens any character devices added by driver module, the reference counter is incremented, then it cannot be unload until all of character devices added by the module are released by applications.
If you can unload your module during transmission of audio data frame, it's apparently a bug of your module.
Does ALSA need handle this case? Or just forbid wave playing during the module load/unload? If this need to be handled, module unload will wait the wave play ending, or just stop the wave play at the first of the load/unload?
All of character devices for a sound card are associated to sound card structure (struct snd_card). Kernel modules should set 'disconnected' state to the structure in its callback for .remove invoked in unloading/unbinding operation. Then appliations receives errors in I/O operations to the character device. The applications are expected to close the character device. When all of character devices are closed, then sound card structure is going to be released.
OK, thank you very much! I will check the reference count in my module.
BR
Yingjiang
Regards
Takashi Sakamoto
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Zhu, Yingjiang