• David Dillow's avatar
    ALSA: pcm_lib: avoid timing jitter in snd_pcm_read/write() · 5daeba34
    David Dillow authored
    When using poll() to wait for the next period -- or avail_min samples --
    one gets a consistent delay for each system call that is usually just a
    little short of the selected period time. However, When using
    snd_pcm_read/write(), one gets a jittery delay that alternates between
    less than a millisecond and approximately two period times. This is
    caused by snd_pcm_lib_{read,write}1() transferring any available samples
    to the user's buffer and adjusting the application pointer prior to
    sleeping to the end of the current period. When the next period
    interrupt occurs, there is then less than avail_min samples remaining to
    be transferred in the period, so we end up sleeping until a second
    period occurs.
    This is solved by using runtime->twake as the number of samples needed
    for a wakeup in addition to selecting the proper wait queue to wake in
    snd_pcm_update_state(). This requires twake to be non-zero when used
    by snd_pcm_lib_{read,write}1() even if avail_min is zero.
    Signed-off-by: default avatarDave Dillow <dave@thedillows.org>
    Signed-off-by: default avatarJaroslav Kysela <perex@perex.cz>