i2c: Document standard fault codes

This is a summary of the most important conventions for use of fault

codes in the I2C/SMBus stack.

A "Fault" is not always an "Error"

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Not all fault reports imply errors; "page faults" should be a familiar

example.  Software often retries idempotent operations after transient

faults.  There may be fancier recovery schemes that are appropriate in

some cases, such as re-initializing (and maybe resetting).  After such

recovery, triggered by a fault report, there is no error.

In a similar way, sometimes a "fault" code just reports one defined

result for an operation ... it doesn't indicate that anything is wrong

at all, just that the outcome wasn't on the "golden path".

In short, your I2C driver code may need to know these codes in order

to respond correctly.  Other code may need to rely on YOUR code reporting

the right fault code, so that it can (in turn) behave correctly.

I2C and SMBus fault codes

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These are returned as negative numbers from most calls, with zero or

some positive number indicating a non-fault return.  The specific

numbers associated with these symbols differ between architectures,

though most Linux systems use <asm-generic/errno*.h> numbering.

Note that the descriptions here are not exhaustive.  There are other

codes that may be returned, and other cases where these codes should

be returned.  However, drivers should not return other codes for these

cases (unless the hardware doesn't provide unique fault reports).

Also, codes returned by adapter probe methods follow rules which are

specific to their host bus (such as PCI, or the platform bus).

EAGAIN

	Returned by I2C adapters when they lose arbitration in master

	transmit mode:  some other master was transmitting different

	data at the same time.

	Also returned when trying to invoke an I2C operation in an

	atomic context, when some task is already using that I2C bus

	to execute some other operation.

EBADMSG

	Returned by SMBus logic when an invalid Packet Error Code byte

	is received.  This code is a CRC covering all bytes in the

	transaction, and is sent before the terminating STOP.  This

	fault is only reported on read transactions; the SMBus slave

	may have a way to report PEC mismatches on writes from the

	host.  Note that even if PECs are in use, you should not rely

	on these as the only way to detect incorrect data transfers.

EBUSY

	Returned by SMBus adapters when the bus was busy for longer

	than allowed.  This usually indicates some device (maybe the

	SMBus adapter) needs some fault recovery (such as resetting),

	or that the reset was attempted but failed.

EINVAL

	This rather vague error means an invalid parameter has been

	detected before any I/O operation was started.  Use a more

	specific fault code when you can.

	One example would be a driver trying an SMBus Block Write

	with block size outside the range of 1-32 bytes.

EIO

	This rather vague error means something went wrong when

	performing an I/O operation.  Use a more specific fault

	code when you can.

ENODEV

	Returned by driver probe() methods.  This is a bit more

	specific than ENXIO, implying the problem isn't with the

	address, but with the device found there.  Driver probes

	may verify the device returns *correct* responses, and

	return this as appropriate.  (The driver core will warn

	about probe faults other than ENXIO and ENODEV.)

ENOMEM

	Returned by any component that can't allocate memory when

	it needs to do so.

ENXIO

	Returned by I2C adapters to indicate that the address phase

	of a transfer didn't get an ACK.  While it might just mean

	an I2C device was temporarily not responding, usually it

	means there's nothing listening at that address.

	Returned by driver probe() methods to indicate that they

	found no device to bind to.  (ENODEV may also be used.)

EOPNOTSUPP

	Returned by an adapter when asked to perform an operation

	that it doesn't, or can't, support.

	For example, this would be returned when an adapter that

	doesn't support SMBus block transfers is asked to execute

	one.  In that case, the driver making that request should

	have verified that functionality was supported before it

	made that block transfer request.

	Similarly, if an I2C adapter can't execute all legal I2C

	messages, it should return this when asked to perform a

	transaction it can't.  (These limitations can't be seen in

	the adapter's functionality mask, since the assumption is

	that if an adapter supports I2C it supports all of I2C.)

EPROTO

	Returned when slave does not conform to the relevant I2C

	or SMBus (or chip-specific) protocol specifications.  One

	case is when the length of an SMBus block data response

	(from the SMBus slave) is outside the range 1-32 bytes.

ETIMEDOUT

	This is returned by drivers when an operation took too much

	time, and was aborted before it completed.

	SMBus adapters may return it when an operation took more

	time than allowed by the SMBus specification; for example,

	when a slave stretches clocks too far.  I2C has no such

	timeouts, but it's normal for I2C adapters to impose some

	arbitrary limits (much longer than SMBus!) too.