• Pierre-Louis Bossart's avatar
    ALSA: usb: refine delay information with USB frame counter · 294c4fb8
    Pierre-Louis Bossart authored
    Existing code only updates the audio delay when URBs were
    submitted/retired. This can introduce an uncertainty of 8ms
    on the number of samples played out with the default settings,
    and a lot more when URBs convey more packets to reduce the
    interrupt rate and power consumption.
    This patch relies on the USB frame counter to reduce the
    uncertainty to less than 2ms worst-case. The delay information
    essentially becomes independent of the URB size and number of
    packets. This should help applications like PulseAudio which
    require accurate audio timing. Clemens Ladisch reported
    a decrease of mplayer's A-V difference from nrpacks down to at
    most 1ms.
    Thanks to Clemens for also pointing out that the implementation
    of frame counters varies between different HCDs. Only the
    8 lowest-bits are used to estimate the delay.
    Signed-off-by: default avatarPierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
    [clemens: changed debug code]
    Signed-off-by: default avatarClemens Ladisch <clemens@ladisch.de>
    Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>