• Takashi Iwai's avatar
    ALSA: usb-audio: Add hw constraint for implicit fb sync · 5a6c3e11
    Takashi Iwai authored
    
    
    In the current code, there is no check at the stream open time whether
    the endpoint is being already used by others.  In the normal
    operations, this shouldn't happen, but in the case of the implicit
    feedback mode, it's a common problem with the full duplex operation,
    because the capture stream is always opened by the playback stream as
    an implicit sync source.
    
    Although we recently introduced the check of such a conflict of
    parameters at the PCM hw_params time, it doesn't give any hint at the
    hw_params itself and just gives the error.  This isn't quite
    comfortable, and it caused problems on many applications.
    
    This patch attempts to make the parameter handling easier by
    introducing the strict hw constraint matching with the counterpart
    stream that is being used.  That said, when an implicit feedback
    playback stream is running before a capture stream is opened, the
    capture stream carries the PCM hw-constraint to allow only the same
    sample rate, format, periods and period frames as the running playback
    stream.  If not opened or there is no conflict of endpoints, the
    behavior remains as same as before.
    
    Note that this kind of "weak link" should work for most cases, but
    this is no concrete solution; e.g. if an application changes the hw
    params multiple times while another stream is opened, this would lead
    to inconsistencies.
    Tested-by: default avatarKeith Milner <kamilner@superlative.org>
    Tested-by: default avatarDylan Robinson <dylan_robinson@motu.com>
    Link: https://lore.kernel.org/r/20201123085347.19667-11-tiwai@suse.de
    
    Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
    5a6c3e11