• Alexander Tsoy's avatar
    ALSA: usb-audio: Improve frames size computation · f0bd62b6
    Alexander Tsoy authored
    
    
    For computation of the the next frame size current value of fs/fps and
    accumulated fractional parts of fs/fps are used, where values are stored
    in Q16.16 format. This is quite natural for computing frame size for
    asynchronous endpoints driven by explicit feedback, since in this case
    fs/fps is a value provided by the feedback endpoint and it's already in
    the Q format. If an error is accumulated over time, the device can
    adjust fs/fps value to prevent buffer overruns/underruns.
    
    But for synchronous endpoints the accuracy provided by these computations
    is not enough. Due to accumulated error the driver periodically produces
    frames with incorrect size (+/- 1 audio sample).
    
    This patch fixes this issue by implementing a different algorithm for
    frame size computation. It is based on accumulating of the remainders
    from division fs/fps and it doesn't accumulate errors over time. This
    new method is enabled for synchronous and adaptive playback endpoints.
    Signed-off-by: default avatarAlexander Tsoy <alexander@tsoy.me>
    Link: https://lore.kernel.org/r/20200424022449.14972-1-alexander@tsoy.me
    
    Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
    f0bd62b6