Re: Suspend/resume Issue on pcm_dmix.c in alsa-lib

[Takashi Iwai <tiwai@suse.de>]

[ it seems that my previous post didn't go out properly, so resent;
  if you've seen already the same, please disregard ]

On Tue, 27 Aug 2024 09:06:39 +0200,
Chancel Liu wrote:
> 
> 
> Hi Takashi Iwai, Jaroslav Kysela
> 
> We found an issue on dmix in alsa-lib when do suspend and resume. It can be
> easily reproduced by following steps:
> 
> 1. Run two dmix clients in parallel. (Only one client doesn’t has such issue)
> 
> ~# aplay xxx1.wav &
> 
> ~# aplay xxx2.wav &
> 
> Here I attach the asound.conf we're using.
> 
> ~# cat /etc/asound.conf
> 
> defaults.pcm.rate_converter "linear"
> 
> pcm.dmix_44100{
> 
>     type dmix
> 
>     ipc_key 5678293
> 
>     ipc_key_add_uid yes
> 
>     slave{
> 
>         pcm "hw:0,0"
> 
>         period_time 40000
> 
>         format S16_LE
> 
>         rate 44100
> 
>         }
> 
> }
> 
> pcm.asymed{
> 
>     type asym
> 
>     playback.pcm "dmix_44100"
> 
>     capture.pcm "dsnoop_44100"
> 
> }
> 
> pcm.!default{
> 
>     type plug
> 
>     route_policy "average"
> 
>     slave.pcm "asymed"
> 
> }
> 
> 2. Let linux enter into suspend and then resume(Repeat this step if not
> reproduced)
> 
> 3. After resume, aplay will get stuck in snd_pcm_wait(). The GDB shows:
> 
> (gdb) bt
> 
> #0  0x0000fffff7da9264 in __GI___poll (fds=fds@entry=0xfffffffff480, nfds=
> nfds@entry=1, timeout=timeout@entry=240)
> 
>     at /usr/src/debug/glibc/2.39+git/sysdeps/unix/sysv/linux/poll.c:41
> 
> #1  0x0000fffff7edf468 in poll (__timeout=240, __nfds=1, __fds=0xfffffffff480)
> 
> #2  snd1_pcm_wait_nocheck (pcm=pcm@entry=0xaaaaaaad2cb0, timeout=240,
> timeout@entry=-10001) at pcm.c:2993
> 
> #3  0x0000fffff7ee54a0 in snd1_pcm_write_areas (pcm=pcm@entry=0xaaaaaaad2cb0,
> areas=areas@entry=0xfffffffff560, offset=<optimized out>, offset@entry=0, size
> =<optimized out>,
> 
>     size@entry=1768, func=func@entry=0xfffff7ef5190
> <snd_pcm_plugin_write_areas>) at pcm.c:7699
> 
> #4  0x0000fffff7ef5020 in snd_pcm_plugin_writei (pcm=0xaaaaaaad2cb0, buffer=
> <optimized out>, size=1768) at pcm_plugin.c:354
> 
> It seems that sometimes after suspend and resume there's no available space
> for data written into buffer. Then aplay keeps stuck in snd_pcm_wait(). I
> checked the hw_ptr of dmix and found that hw_ptr is always 0 after resume.
> 
> I don't have a solution now so I turn to you for help. The version of alsa-lib
> is v1.2.11. Could you please help check it?

I tried your setup but I couldn't reproduce the issue locally with my
laptop and HD-audio device.  Possibly depending on the kernel driver?

In the case of dmix, it's a poll() against the PCM slave timer.  So
it doesn't take care of suspend/resume state unlike the real PCM
device.  OTOH, the timer device should send notification events at
suspend/resume, and it should trigger the poll wakeup, too.

Does poll() return after the suspend/resume once but falls into a loop
due to revents being unset?  Or it's stuck and never returns at
suspend/resume?


thanks,

Takashi