linux-kernel - [PATCH RFC 2/7] i2c: mux: add idle_state property to i2c_mux

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Message-Id: <20250922-i2c-mux-v1-2-28c94a610930@gmail.com>
Date: Mon, 22 Sep 2025 08:20:57 +0200
From: Marcus Folkesson <marcus.folkesson@...il.com>
To: Wolfram Sang <wsa+renesas@...g-engineering.com>, 
 Peter Rosin <peda@...ntia.se>, 
 Michael Hennerich <michael.hennerich@...log.com>, 
 Bartosz Golaszewski <brgl@...ev.pl>, Andi Shyti <andi.shyti@...nel.org>
Cc: linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-arm-kernel@...ts.infradead.org, 
 Marcus Folkesson <marcus.folkesson@...il.com>
Subject: [PATCH RFC 2/7] i2c: mux: add idle_state property to i2c_mux_core

Muxes treat their channels differently when idle.
Let the mux core have this information to make it available for
internal use.
Reuse the same state values used by CONFIG_MULTIPLEXER.

Possible idle states are:
- MUX_IDLE_AS_IS: Leave channels as is when idle
- MUX_IDLE_DISCONNECT: Disconnect channel (set HiZ when idle)
- <n>: Enable channel n when idle

Default value is set to MUX_IDLE_AS_IS.

Signed-off-by: Marcus Folkesson <marcus.folkesson@...il.com>
---
 drivers/i2c/i2c-mux.c   |  1 +
 include/linux/i2c-mux.h | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/drivers/i2c/i2c-mux.c b/drivers/i2c/i2c-mux.c
index 4d8690981a55dc0e1b35454971923791e6ed9f7f..5caa927c0caae512af029f0d1ae9b7f845ba3f6f 100644
--- a/drivers/i2c/i2c-mux.c
+++ b/drivers/i2c/i2c-mux.c
@@ -250,6 +250,7 @@ struct i2c_mux_core *i2c_mux_alloc(struct i2c_adapter *parent,
 	muxc->select = select;
 	muxc->deselect = deselect;
 	muxc->max_adapters = max_adapters;
+	muxc->idle_state = MUX_IDLE_AS_IS;
 
 	return muxc;
 }
diff --git a/include/linux/i2c-mux.h b/include/linux/i2c-mux.h
index 1784ac7afb116ce64e9e9fca1ee127c2bbbd2fb9..9e230ad69377b74f9ec809c40ccffd95538c4595 100644
--- a/include/linux/i2c-mux.h
+++ b/include/linux/i2c-mux.h
@@ -13,6 +13,7 @@
 
 #ifdef __KERNEL__
 
+#include <dt-bindings/mux/mux.h>
 #include <linux/bitops.h>
 
 struct i2c_mux_core {
@@ -22,6 +23,12 @@ struct i2c_mux_core {
 	unsigned int arbitrator:1;
 	unsigned int gate:1;
 
+	/*
+	 * The mux controller state to use when inactive, or one
+	 * of MUX_IDLE_AS_IS and MUX_IDLE_DISCONNECT.
+	 */
+	int idle_state;
+
 	void *priv;
 
 	int (*select)(struct i2c_mux_core *, u32 chan_id);
@@ -38,6 +45,20 @@ struct i2c_mux_core *i2c_mux_alloc(struct i2c_adapter *parent,
 				   int (*select)(struct i2c_mux_core *, u32),
 				   int (*deselect)(struct i2c_mux_core *, u32));
 
+/*
+ * Mux drivers may only change idle_state, and may only do so
+ * between allocation and registration of the mux controller.
+ */
+static inline void i2c_mux_set_idle_state(struct i2c_mux_core *muxc, int state)
+{
+	muxc->idle_state = state;
+}
+
+static inline int i2c_mux_idle_state(struct i2c_mux_core *muxc)
+{
+	return muxc->idle_state;
+}
+
 /* flags for i2c_mux_alloc */
 #define I2C_MUX_LOCKED     BIT(0)
 #define I2C_MUX_ARBITRATOR BIT(1)

-- 
2.50.1