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Based on kernel version 3.16. Page generated on 2014-08-06 21:39 EST.

1	This is a summary of the most important conventions for use of fault
2	codes in the I2C/SMBus stack.
3	
4	
5	A "Fault" is not always an "Error"
6	----------------------------------
7	Not all fault reports imply errors; "page faults" should be a familiar
8	example.  Software often retries idempotent operations after transient
9	faults.  There may be fancier recovery schemes that are appropriate in
10	some cases, such as re-initializing (and maybe resetting).  After such
11	recovery, triggered by a fault report, there is no error.
12	
13	In a similar way, sometimes a "fault" code just reports one defined
14	result for an operation ... it doesn't indicate that anything is wrong
15	at all, just that the outcome wasn't on the "golden path".
16	
17	In short, your I2C driver code may need to know these codes in order
18	to respond correctly.  Other code may need to rely on YOUR code reporting
19	the right fault code, so that it can (in turn) behave correctly.
20	
21	
22	I2C and SMBus fault codes
23	-------------------------
24	These are returned as negative numbers from most calls, with zero or
25	some positive number indicating a non-fault return.  The specific
26	numbers associated with these symbols differ between architectures,
27	though most Linux systems use <asm-generic/errno*.h> numbering.
28	
29	Note that the descriptions here are not exhaustive.  There are other
30	codes that may be returned, and other cases where these codes should
31	be returned.  However, drivers should not return other codes for these
32	cases (unless the hardware doesn't provide unique fault reports).
33	
34	Also, codes returned by adapter probe methods follow rules which are
35	specific to their host bus (such as PCI, or the platform bus).
36	
37	
38	EAGAIN
39		Returned by I2C adapters when they lose arbitration in master
40		transmit mode:  some other master was transmitting different
41		data at the same time.
42	
43		Also returned when trying to invoke an I2C operation in an
44		atomic context, when some task is already using that I2C bus
45		to execute some other operation.
46	
47	EBADMSG
48		Returned by SMBus logic when an invalid Packet Error Code byte
49		is received.  This code is a CRC covering all bytes in the
50		transaction, and is sent before the terminating STOP.  This
51		fault is only reported on read transactions; the SMBus slave
52		may have a way to report PEC mismatches on writes from the
53		host.  Note that even if PECs are in use, you should not rely
54		on these as the only way to detect incorrect data transfers.
55	
56	EBUSY
57		Returned by SMBus adapters when the bus was busy for longer
58		than allowed.  This usually indicates some device (maybe the
59		SMBus adapter) needs some fault recovery (such as resetting),
60		or that the reset was attempted but failed.
61	
62	EINVAL
63		This rather vague error means an invalid parameter has been
64		detected before any I/O operation was started.  Use a more
65		specific fault code when you can.
66	
67	EIO
68		This rather vague error means something went wrong when
69		performing an I/O operation.  Use a more specific fault
70		code when you can.
71	
72	ENODEV
73		Returned by driver probe() methods.  This is a bit more
74		specific than ENXIO, implying the problem isn't with the
75		address, but with the device found there.  Driver probes
76		may verify the device returns *correct* responses, and
77		return this as appropriate.  (The driver core will warn
78		about probe faults other than ENXIO and ENODEV.)
79	
80	ENOMEM
81		Returned by any component that can't allocate memory when
82		it needs to do so.
83	
84	ENXIO
85		Returned by I2C adapters to indicate that the address phase
86		of a transfer didn't get an ACK.  While it might just mean
87		an I2C device was temporarily not responding, usually it
88		means there's nothing listening at that address.
89	
90		Returned by driver probe() methods to indicate that they
91		found no device to bind to.  (ENODEV may also be used.)
92	
93	EOPNOTSUPP
94		Returned by an adapter when asked to perform an operation
95		that it doesn't, or can't, support.
96	
97		For example, this would be returned when an adapter that
98		doesn't support SMBus block transfers is asked to execute
99		one.  In that case, the driver making that request should
100		have verified that functionality was supported before it
101		made that block transfer request.
102	
103		Similarly, if an I2C adapter can't execute all legal I2C
104		messages, it should return this when asked to perform a
105		transaction it can't.  (These limitations can't be seen in
106		the adapter's functionality mask, since the assumption is
107		that if an adapter supports I2C it supports all of I2C.)
108	
109	EPROTO
110		Returned when slave does not conform to the relevant I2C
111		or SMBus (or chip-specific) protocol specifications.  One
112		case is when the length of an SMBus block data response
113		(from the SMBus slave) is outside the range 1-32 bytes.
114	
115	ETIMEDOUT
116		This is returned by drivers when an operation took too much
117		time, and was aborted before it completed.
118	
119		SMBus adapters may return it when an operation took more
120		time than allowed by the SMBus specification; for example,
121		when a slave stretches clocks too far.  I2C has no such
122		timeouts, but it's normal for I2C adapters to impose some
123		arbitrary limits (much longer than SMBus!) too.
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