[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...@nxp.com>
---
 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