viafb: Only establish i2c busses on ports that always had them

...otherwise it seems we run into conflicts with shadowy other users which
don't expect to see i2c taking control of ports it never used to do
anything with.
Reported-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

drivers/video/via/via_i2c.c

@@ -170,13 +170,18 @@ static int create_i2c_bus(struct i2c_adapter *adapter,
 	return i2c_bit_add_bus(adapter);
 }
 
+/*
+ * By default, we only activate busses on ports 2c and 31 to avoid
+ * conflicts with other possible users; that default can be changed
+ * below.
+ */
 static struct via_i2c_adap_cfg adap_configs[] = {
-	[VIA_I2C_ADAP_26]	= { VIA_I2C_I2C,  VIASR, 0x26 },
-	[VIA_I2C_ADAP_31]	= { VIA_I2C_I2C,  VIASR, 0x31 },
-	[VIA_I2C_ADAP_25]	= { VIA_I2C_GPIO, VIASR, 0x25 },
-	[VIA_I2C_ADAP_2C]	= { VIA_I2C_GPIO, VIASR, 0x2c },
-	[VIA_I2C_ADAP_3D]	= { VIA_I2C_GPIO, VIASR, 0x3d },
-	{ 0, 0, 0 }
+	[VIA_I2C_ADAP_26]	= { VIA_I2C_I2C,  VIASR, 0x26, 0 },
+	[VIA_I2C_ADAP_31]	= { VIA_I2C_I2C,  VIASR, 0x31, 1 },
+	[VIA_I2C_ADAP_25]	= { VIA_I2C_GPIO, VIASR, 0x25, 0 },
+	[VIA_I2C_ADAP_2C]	= { VIA_I2C_GPIO, VIASR, 0x2c, 1 },
+	[VIA_I2C_ADAP_3D]	= { VIA_I2C_GPIO, VIASR, 0x3d, 0 },
+	{ 0, 0, 0, 0 }
 };
 
 int viafb_create_i2c_busses(struct viafb_par *viapar)
@@ -189,6 +194,8 @@ int viafb_create_i2c_busses(struct viafb_par *viapar)
 
 		if (adap_cfg->type == 0)
 			break;
+		if (!adap_cfg->is_active)
+			continue;
 
 		ret = create_i2c_bus(&i2c_stuff->adapter,
 				     &i2c_stuff->algo, adap_cfg,

drivers/video/via/via_i2c.h

@@ -35,6 +35,7 @@ struct via_i2c_adap_cfg {
 	enum via_i2c_type	type;
 	u_int16_t		io_port;
 	u_int8_t		ioport_index;
+	u8			is_active;
 };
 
 struct via_i2c_stuff {