Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 28 insertions, 0 deletions

diff --git a/Documentation/i2c/ten-bit-addresses b/Documentation/i2c/ten-bit-addresses
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+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).
+To avoid ambiguity, the user sees 10 bit addresses mapped to a different
+address space, namely 0xa000-0xa3ff. The leading 0xa (= 10) represents the
+10 bit mode. This is used for creating device names in sysfs. It is also
+needed when instantiating 10 bit devices via the new_device file in sysfs.
+
+I2C messages to and from 10-bit address devices have a different format.
+See the I2C specification for the details.
+
+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.