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i2c: Fix device name for 10-bit slave address

10-bit addresses overlap with traditional 7-bit addresses, leading in
device name collisions. Add an arbitrary offset to 10-bit addresses to
prevent this collision. The offset was chosen so that the address is
still easily recognizable.

Signed-off-by: Jean Delvare <>
Acked-by: Wolfram Sang <>
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1 parent cc6bcf7 commit cbb44514048a250647c6c6b3df27ff62cb71f7d5 Jean Delvare committed with Jean Delvare Nov 23, 2011
Showing with 22 additions and 18 deletions.
  1. +19 −17 Documentation/i2c/ten-bit-addresses
  2. +3 −1 drivers/i2c/i2c-core.c
@@ -1,22 +1,24 @@
The I2C protocol knows about two kinds of device addresses: normal 7 bit
addresses, and an extended set of 10 bit addresses. The sets of addresses
do not intersect: the 7 bit address 0x10 is not the same as the 10 bit
-address 0x10 (though a single device could respond to both of them). You
-select a 10 bit address by adding an extra byte after the address
- S Addr7 Rd/Wr ....
- S 11110 Addr10 Rd/Wr
-S is the start bit, Rd/Wr the read/write bit, and if you count the number
-of bits, you will see the there are 8 after the S bit for 7 bit addresses,
-and 16 after the S bit for 10 bit addresses.
+address 0x10 (though a single device could respond to both of them).
-WARNING! The current 10 bit address support is EXPERIMENTAL. There are
-several places in the code that will cause SEVERE PROBLEMS with 10 bit
-addresses, even though there is some basic handling and hooks. Also,
-almost no supported adapter handles the 10 bit addresses correctly.
+I2C messages to and from 10-bit address devices have a different format.
+See the I2C specification for the details.
-As soon as a real 10 bit address device is spotted 'in the wild', we
-can and will add proper support. Right now, 10 bit address devices
-are defined by the I2C protocol, but we have never seen a single device
-which supports them.
+The current 10 bit address support is minimal. It should work, however
+you can expect some problems along the way:
+* Not all bus drivers support 10-bit addresses. Some don't because the
+ hardware doesn't support them (SMBus doesn't require 10-bit address
+ support for example), some don't because nobody bothered adding the
+ code (or it's there but not working properly.) Software implementation
+ (i2c-algo-bit) is known to work.
+* Some optional features do not support 10-bit addresses. This is the
+ case of automatic detection and instantiation of devices by their,
+ drivers, for example.
+* Many user-space packages (for example i2c-tools) lack support for
+ 10-bit addresses.
+Note that 10-bit address devices are still pretty rare, so the limitations
+listed above could stay for a long time, maybe even forever if nobody
+needs them to be fixed.
@@ -539,8 +539,10 @@ i2c_new_device(struct i2c_adapter *adap, struct i2c_board_info const *info)
client->dev.type = &i2c_client_type;
client->dev.of_node = info->of_node;
+ /* For 10-bit clients, add an arbitrary offset to avoid collisions */
dev_set_name(&client->dev, "%d-%04x", i2c_adapter_id(adap),
- client->addr);
+ client->addr | ((client->flags & I2C_CLIENT_TEN)
+ ? 0xa000 : 0));
status = device_register(&client->dev);
if (status)
goto out_err;

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