• Takashi Iwai's avatar
    ALSA: pcm - Optimize the call of snd_pcm_update_hw_ptr() in read/write loop · 0910c216
    Takashi Iwai authored
    
    
    In the PCM read/write loop, the driver calls snd_pcm_update_hw_ptr()
    at each time at the beginning of the loop.  Russell King reported that
    this hogs CPU significantly.
    
    The current code assumes that the pointer callback is very fast and
    cheap, also not too much fine grained.  It's not true in all cases.
    When the pointer advances short samples while the read/write copy has
    been performed, the driver updates the hw_ptr and gets avail > 0
    again.  Then it tries to read/write these small chunks.  This repeats
    until the avail really gets to zero.
    
    For avoiding this situation, a simple workaround is to call
    snd_pcm_update_hw_ptr() only once at starting the loop, assuming that
    the read/write copy is performed fast enough.  If the available count
    becomes short, it goes to snd_pcm_wait_avail() anyway, and this
    processes right.
    
    Tested-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
    0910c216