[alsa-devel] Question about sound device release timing

25 Aug 2017


      Hi Takashi-san, Mark
I noticed snd_soc_dapm_stream_event() is called strange timing
if I unbind sound device during playing
> aplay xxx.wav &
    > echo xxxx > /sys/bus/platform/drivers/xxx/unbind 
    ...
    > Unable to handle kernel paging request at virtual address dead000000000220
    > Mem abort info:
    (snip)
    > PC is at soc_dapm_dai_stream_event.isra.14+0x20/0xd0
    > LR is at snd_soc_dapm_stream_event+0x74/0xa8
    (snip)
    > [<ffff000008715610>] soc_dapm_dai_stream_event.isra.14+0x20/0xd0
    > [<ffff00000871989c>] snd_soc_dapm_stream_event+0x74/0xa8
    > [<ffff00000871b23c>] close_delayed_work+0x3c/0x50
    > [<ffff0000080bbd6c>] process_one_work+0x1ac/0x318
    > [<ffff0000080bbf20>] worker_thread+0x48/0x420
    > [<ffff0000080c201c>] kthread+0xfc/0x128
    > [<ffff0000080842f0>] ret_from_fork+0x10/0x18
    > Code: b40005c9 a9bf7bfd 91048120 910003fd (f9409121) 
    > ---[ end trace 6739a0ea1013e0b2 ]---
This snd_soc_dapm_stream_event() was called from close_delayed_work()
and it was set on soc_pcm_close()
static int soc_pcm_close(struct snd_pcm_substream *substream)
    {
    	...
    	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
    		if (snd_soc_runtime_ignore_pmdown_time(rtd)) {
    			...
    		} else {
    			/* start delayed pop wq here for playback streams */
    			rtd->pop_wait = 1;
=>				queue_delayed_work(system_power_efficient_wq,
    					   &rtd->delayed_work,
    					   msecs_to_jiffies(rtd->pmdown_time));
    		}
    	...
    }
But, this soc_pcm_close() is called *after* flush_delayed_work() of
soc_cleanup_card_resources().
It seems this close function seems called from snd_card_free()
and this rtd will be freed on soc_remove_pcm_runtimes().
static int soc_cleanup_card_resources(struct snd_soc_card *card)
{
    ...
/* make sure any delayed work runs */
    list_for_each_entry(rtd, &card->rtd_list, list) {
=>		flush_delayed_work(&rtd->delayed_work);
    }
/* free the ALSA card at first; this syncs with pending operations */
=>	snd_card_free(card->snd_card);
    ...
=>	soc_remove_pcm_runtimes(card);
    ...
}
Because of this, delayed called close_delayed_work() will call
soc_dapm_dai_stream_event() and it access to already freed rtd,
and kernel will die.
I don't know why, but it seems this close is wokring on other thread (?),
but I'm not sure.
Q1. does this close function run on other thread ?
Q2. I can avoid this delayed close by setting rtd->pmdown_time = 0;
    before snd_card_free(). but is this correct approach ?
I can avoid this delayed calling close_delayed_work() by local patch,
but then, re-bind doesn't work correctly.
/*
     * custom kernel for avoiding close_delayed_work()
     */
    > aplay xxx.wav &
    > echo xxxx > /sys/bus/platform/drivers/rcar_sound/unbind
    /* unbind codec and card drivers here */
    >
    /*
     * try re-bind
     */
    /* re-bind codec and card drivers here */
    > echo xxxx > /sys/bus/platform/drivers/rcar_sound/bind 
    rcar_sound ec500000.sound: probed
    asoc-simple-card sound: ak4613-hifi <-> ec500000.sound mapping ok
    >
    > aplay xxx.wav 
    Playing WAVE '/home/Calm_16bit_48k.wav' : Signed 16 bit Little Endian, Rate 48000 Hz, Stereo
    ALSA lib pcm_params.c:2162:(snd1_pcm_hw_refine_slave) Slave PCM not usable
    aplay: set_params:1204: Broken configuration for this PCM: no configurations available
Do you know why or have hint to solve these issue ?
Best regards
---
Kuninori Morimoto