ASoC: basic support for configuring bus keepers

James Calligeros <jcalligeros99@gmail.com> says:

This series introduces some infrastructure to allow platform drivers
to specify what a DAI should be doing when it is not active on the
bus. The primary use case for this is configuring bus keepers which
may be integrated into various codecs. The instigating use case for
this functionality is an interesting bus topology on Apple Silicon
laptops with multiple codecs.

Most Apple Silicon laptops have six codecs split into groups of
three, driving a pair of dual opposed woofers and a tweeter for
L/R stereo sound. These codecs report the voltage and current across
their connected voice coils back to the SoC via the SDOUT pin,
represented as PCM data sent via configurable TDM slots. This data is
used in conjunction with the connected speaker's Thiele/Small Parameters
to ensure that the speaker is not being driven to levels that would
permanently damage them. This is integrated into CoreAudio on macOS.
speakersafetyd[1] handles this for Linux.

All of the codec SDOUT pins are attached to a single receiver port
on the SoC's I2S peripheral, however are split across two physical
data lines (one each for the left and right codec groups). The receiver
has an OR gate in front of it, which is used to sum the two lines.

If at any point a codec is trying to transmit data, and the "opposite"
line ends up floating high, the transmitting codec's data will be
corrupted. We need to guarantee that the idle line stays idle.

In the downstream Asahi Linux kernel[2], we set up one codec in each
group to zero-fill or pull down its line while a codec on the opposite
line is actively transmitting. This is done entirely in the codec
driver, however this approach is over-fit for this one use case. This
sort of functionality may also be of use for other hardware, so following
previous mailing list discussions[3], I have tried to expose the
functionality in a more configurable and generic way.

I have integrated this approach into our downstream platform driver
and select Devicetrees as an example of how this mechanism is intended
to be used[4].

[1] https://github.com/AsahiLinux/speakersafetyd
[2] https://github.com/AsahiLinux/linux/tree/bits/070-audio
[3] https://lore.kernel.org/asahi/20250227-apple-codec-changes-v3-17-cbb130030acf@gmail.com/
[4] https://github.com/chadmed/tree/tdm-revised2

Link: https://patch.msgid.link/20260301-tdm-idle-slots-v3-0-c6ac5351489a@gmail.com

1 files changed, 22 insertions, 0 deletions

diff --git a/include/sound/soc-dai.h b/include/sound/soc-dai.h
index 224396927aef3..6a42812bba8ca 100644
--- a/include/sound/soc-dai.h
+++ b/include/sound/soc-dai.h
@@ -53,6 +53,21 @@ struct snd_compr_stream;
 #define SND_SOC_POSSIBLE_DAIFMT_PDM		(1 << SND_SOC_DAI_FORMAT_PDM)
 
 /*
+ * DAI TDM slot idle modes
+ *
+ * Describes a CODEC/CPU's behaviour when not actively receiving or
+ * transmitting on a given TDM slot. NONE is undefined behaviour.
+ * Add new modes to the end.
+ */
+#define SND_SOC_DAI_TDM_IDLE_NONE	0
+#define SND_SOC_DAI_TDM_IDLE_OFF	1
+#define SND_SOC_DAI_TDM_IDLE_ZERO	2
+#define SND_SOC_DAI_TDM_IDLE_PULLDOWN	3
+#define SND_SOC_DAI_TDM_IDLE_HIZ	4
+#define SND_SOC_DAI_TDM_IDLE_PULLUP	5
+#define SND_SOC_DAI_TDM_IDLE_DRIVE_HIGH	6
+
+/*
  * DAI Clock gating.
  *
  * DAI bit clocks can be gated (disabled) when the DAI is not
@@ -181,6 +196,10 @@ int snd_soc_dai_set_fmt(struct snd_soc_dai *dai, unsigned int fmt);
 int snd_soc_dai_set_tdm_slot(struct snd_soc_dai *dai,
 	unsigned int tx_mask, unsigned int rx_mask, int slots, int slot_width);
 
+int snd_soc_dai_set_tdm_idle(struct snd_soc_dai *dai,
+			     unsigned int tx_mask, unsigned int rx_mask,
+			     int tx_mode, int rx_mode);
+
 int snd_soc_dai_set_channel_map(struct snd_soc_dai *dai,
 	unsigned int tx_num, const unsigned int *tx_slot,
 	unsigned int rx_num, const unsigned int *rx_slot);
@@ -297,6 +316,9 @@ struct snd_soc_dai_ops {
 	int (*set_tdm_slot)(struct snd_soc_dai *dai,
 		unsigned int tx_mask, unsigned int rx_mask,
 		int slots, int slot_width);
+	int (*set_tdm_idle)(struct snd_soc_dai *dai,
+			    unsigned int tx_mask, unsigned int rx_mask,
+			    int tx_mode, int rx_mode);
 	int (*set_channel_map)(struct snd_soc_dai *dai,
 		unsigned int tx_num, const unsigned int *tx_slot,
 		unsigned int rx_num, const unsigned int *rx_slot);