• Takashi Sakamoto's avatar
    ALSA: firewire: process packets in 'struct snd_pcm_ops.ack' callback · 875becf8
    Takashi Sakamoto authored
    In recent commit for ALSA PCM core, some arrangement is done for
    'struct snd_pcm_ops.ack' callback. This is called when appl_ptr is
    explicitly moved in intermediate buffer for PCM frames, except for
    some cases described later.
    
    For drivers in ALSA firewire stack, usage of this callback has a merit to
    reduce latency between time of PCM frame queueing and handling actual
    packets in recent isochronous cycle, because no need to wait for software
    IRQ context from isochronous context of OHCI 1394.
    
    If this works well in a case that mapped page frame is used for the
    intermediate buffer, user process should execute some commands for ioctl(2)
    to tell the number of handled PCM frames in the intermediate buffer just
    after handling them. Therefore, at present, with a combination of below
    conditions, this doesn't work as expected and user process should wait for
    the software IRQ context as usual:
     - when ALSA PCM core judges page frame mapping is available for status
       data (struct snd_pcm_mmap_status) and control data
       (struct snd_pcm_mmap_control).
     - user process handles PCM frames by loop just with 'snd_pcm_mmap_begin()'
       and 'snd_pcm_mmap_commit()'.
     - user process uses PCM hw plugin in alsa-lib to operate I/O without
       'sync_ptr_ioctl' option.
    
    Unfortunately, major use case include these three conditions.
    Signed-off-by: 's avatarTakashi Sakamoto <o-takashi@sakamocchi.jp>
    Signed-off-by: 's avatarTakashi Iwai <tiwai@suse.de>
    875becf8
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