ASoC: rt5677: Add a PCM device for streaming hotword via SPI

This patch implements a PCM interface for streaming hotword
phrases over SPI. Userspace can open the PCM device at anytime.
The stream is blocked when no hotword is detected. The mic
audio buffer on the DSP is a ~128KByte ring buffer that holds
~4sec of audio samples recorded from the DMIC (S16_LE, mono,
16KHz). After a hotword is detected, previous 2 seconds of audio
(containing the detected hotword) is streamed first, then live
capture continues until userspace closes the PCM stream.

When transferring, copy one period at a time then call
snd_pcm_period_elapsed(). This reduces the latency of transferring
the initial ~2sec of audio after hotword detect since audio samples
are available for userspace earlier.
Signed-off-by: Ben Zhang <benzh@chromium.org>
Signed-off-by: Curtis Malainey <cujomalainey@chromium.org>
Link: https://lore.kernel.org/r/20191018200449.141123-2-cujomalainey@chromium.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/rt5677-spi.h

@@ -12,5 +12,6 @@
 int rt5677_spi_read(u32 addr, void *rxbuf, size_t len);
 int rt5677_spi_write(u32 addr, const void *txbuf, size_t len);
 int rt5677_spi_write_firmware(u32 addr, const struct firmware *fw);
+void rt5677_spi_hotword_detected(void);
 
 #endif /* __RT5677_SPI_H__ */