• Paul Mackerras's avatar
    [ALSA] aoa i2sbus: Stop Apple i2s DMA gracefully · 547ac2ae
    Paul Mackerras authored
    This fixes the problem of getting extra bytes inserted at the
    beginning of a recording when using the Apple i2s interface and DBDMA
    controller.  It turns out that we can't just abort the DMA; we have to
    let it stop at the end of a command, and then wait for the S7 bit to
    be set before turning off the DBDMA controller.  Doing that for
    playback doesn't seem to be necessary, but doesn't hurt either.
    We use the technique used by the Darwin driver: make each transfer
    command branch to a stop command if the S0 status bit is set.  Thus we
    can ask the DMA controller to stop at the end of the current command
    by setting S0.
    The interrupt routine now looks at and clears the status word of the
    DBDMA command ring.  This is necessary so it can know when the DBDMA
    controller has seen that S0 is set, and so when it should look for the
    DBDMA controller being stopped and S7 being set.  This also ended up
    simplifying the calculation in i2sbus_pcm_pointer.
    Tested on a 15 inch albook.
    [Addition by Johannes]
    I modified this patch and added the suspend/resume bits to it to get my
    powermac into a decent state when playing sound across suspend to disk
    that has a different bitrate from what the firmware programs the
    hardware to.
    I also added the SNDRV_PCM_INFO_JOINT_DUPLEX flag because it seemed the
    right thing to do and I was looking at the info stuff.
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    Signed-off-by: default avatarJohannes Berg <johannes@sipsolutions.net>
    Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
    Signed-off-by: default avatarJaroslav Kysela <perex@suse.cz>
    547ac2ae
i2sbus-core.c 11.9 KB