[PATCH] i2c: pca954x: Add property to skip disabling PCA954x MUX device

[Biwen Li]

On some Layerscape boards like LS2085ARDB and LS2088ARDB,
input pull-up resistors on PCA954x MUX device are missing on board,
So, if MUX are disabled after powered-on, input lines will float
leading to incorrect functionality.

Hence, PCA954x MUX device should never be turned-off after
power-on.

Add property to skip disabling PCA954x MUX device
if device tree contains "i2c-mux-never-disable"
for PCA954x device node.

Errata ID: E-00013 on board LS2085ARDB and LS2088ARDB
(Board revision found on Rev.B, Rev.C and Rev.D)

Signed-off-by: Biwen Li <biwen.li@xxxxxxx>
---
 drivers/i2c/muxes/i2c-mux-pca954x.c | 33 +++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/drivers/i2c/muxes/i2c-mux-pca954x.c b/drivers/i2c/muxes/i2c-mux-pca954x.c
index 923aa3a5a3dc..ea8aca54d572 100644
--- a/drivers/i2c/muxes/i2c-mux-pca954x.c
+++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
@@ -93,6 +93,7 @@ struct pca954x {
 	struct irq_domain *irq;
 	unsigned int irq_mask;
 	raw_spinlock_t lock;
+	u8 disable_mux;		/* do not disable mux if val not 0 */
 };
 
 /* Provide specs for the PCA954x types we know about */
@@ -258,6 +259,11 @@ static int pca954x_deselect_mux(struct i2c_mux_core *muxc, u32 chan)
 	struct i2c_client *client = data->client;
 	s8 idle_state;
 
+	if (data->disable_mux != 0) {
+		data->last_chan = data->chip->nchans;
+		return pca954x_reg_write(muxc->parent, client, data->disable_mux);
+	}
+
 	idle_state = READ_ONCE(data->idle_state);
 	if (idle_state >= 0)
 		/* Set the mux back to a predetermined channel */
@@ -462,16 +468,32 @@ static int pca954x_probe(struct i2c_client *client,
 		}
 	}
 
+	/* Errata ID E-00013 on board LS2088ARDB and LS2088ARDB:
+	 * The point here is that you must not disable a mux if there
+	 * are no pullups on the input or you mess up the I2C. This
+	 * needs to be put into the DTS really as the kernel cannot
+	 * know this otherwise.
+	 */
+
+	data->disable_mux = np &&
+		of_property_read_bool(np, "i2c-mux-never-disable") &&
+		data->chip->muxtype == pca954x_ismux ?
+		data->chip->enable : 0;
+
 	/* Write the mux register at addr to verify
 	 * that the mux is in fact present. This also
 	 * initializes the mux to disconnected state.
 	 */
-	if (i2c_smbus_write_byte(client, 0) < 0) {
+	if (i2c_smbus_write_byte(client, data->disable_mux) < 0) {
 		dev_warn(dev, "probe failed\n");
 		return -ENODEV;
 	}
 
-	data->last_chan = 0;		   /* force the first selection */
+	if (data->disable_mux != 0)
+		data->last_chan = data->chip->nchans;
+	else
+		data->last_chan = 0;               /* force the first selection */
+
 	data->idle_state = MUX_IDLE_AS_IS;
 
 	idle_disconnect_dt = np &&
@@ -531,8 +553,11 @@ static int pca954x_resume(struct device *dev)
 	struct i2c_mux_core *muxc = i2c_get_clientdata(client);
 	struct pca954x *data = i2c_mux_priv(muxc);
 
-	data->last_chan = 0;
-	return i2c_smbus_write_byte(client, 0);
+	if (data->disable_mux != 0)
+		data->last_chan = data->chip->nchans;
+	else
+		data->last_chan = 0;
+	return i2c_smbus_write_byte(client, data->disable_mux);
 }
 #endif
 
-- 
2.17.1