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Commit cbb44514 authored by Jean Delvare's avatar Jean Delvare Committed by Jean Delvare
Browse files

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: default avatarJean Delvare <khali@linux-fr.org>
Acked-by: default avatarWolfram Sang <w.sang@pengutronix.de>
parent cc6bcf7d

Documentation/i2c/ten-bit-addresses

+19 −17
Original line number Diff line number Diff line 
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

byte:

  S Addr7 Rd/Wr ....

becomes

  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.

drivers/i2c/i2c-core.c

+3 −1
Original line number Diff line number Diff line
@@ -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;