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"
  ----------------------------------
  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
  -------------------------
  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.