When we disable rewinds, then the .ack can be used to program SPIB
with the application pointer, which allows the HDaudio DMA to save
power by opportunistically bursting data transfers when the path to
memory is enabled (and conversely to shut it down when there are no
transfer requests).
The SPIB register can only be programmed with incremental values with
wrap-around after the DMA RUN bits are set. For simplicity, we set the
INFO_NO_REWINDS flag in the .open callback when we already need to
program the SNDRV_PCM_INFO_SYNC_APPLPTR flag.
Rewinds are not used by many applications. One notable application
using rewinds is PulseAudio. Practical experiments with
Ubuntu/PulseAudio default settings did not show any audible issues,
but the user may hear volume changes and notification with a delay,
depending on the size of the ring buffer and latency constraints.
The choice of disabling rewinds is exposed as a kernel parameter and
not a Kconfig option to avoid any undesirable side-effects.
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Co-developed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20211119230852.206310-5-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>
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