Root/Documentation/i2c/fault-codes

1This is a summary of the most important conventions for use of fault
2codes in the I2C/SMBus stack.
3
4
5A "Fault" is not always an "Error"
6----------------------------------
7Not all fault reports imply errors; "page faults" should be a familiar
8example. Software often retries idempotent operations after transient
9faults. There may be fancier recovery schemes that are appropriate in
10some cases, such as re-initializing (and maybe resetting). After such
11recovery, triggered by a fault report, there is no error.
12
13In a similar way, sometimes a "fault" code just reports one defined
14result for an operation ... it doesn't indicate that anything is wrong
15at all, just that the outcome wasn't on the "golden path".
16
17In short, your I2C driver code may need to know these codes in order
18to respond correctly. Other code may need to rely on YOUR code reporting
19the right fault code, so that it can (in turn) behave correctly.
20
21
22I2C and SMBus fault codes
23-------------------------
24These are returned as negative numbers from most calls, with zero or
25some positive number indicating a non-fault return. The specific
26numbers associated with these symbols differ between architectures,
27though most Linux systems use <asm-generic/errno*.h> numbering.
28
29Note that the descriptions here are not exhaustive. There are other
30codes that may be returned, and other cases where these codes should
31be returned. However, drivers should not return other codes for these
32cases (unless the hardware doesn't provide unique fault reports).
33
34Also, codes returned by adapter probe methods follow rules which are
35specific to their host bus (such as PCI, or the platform bus).
36
37
38EAGAIN
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
47EBADMSG
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
56EBUSY
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
62EINVAL
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    One example would be a driver trying an SMBus Block Write
68    with block size outside the range of 1-32 bytes.
69
70EIO
71    This rather vague error means something went wrong when
72    performing an I/O operation. Use a more specific fault
73    code when you can.
74
75ENODEV
76    Returned by driver probe() methods. This is a bit more
77    specific than ENXIO, implying the problem isn't with the
78    address, but with the device found there. Driver probes
79    may verify the device returns *correct* responses, and
80    return this as appropriate. (The driver core will warn
81    about probe faults other than ENXIO and ENODEV.)
82
83ENOMEM
84    Returned by any component that can't allocate memory when
85    it needs to do so.
86
87ENXIO
88    Returned by I2C adapters to indicate that the address phase
89    of a transfer didn't get an ACK. While it might just mean
90    an I2C device was temporarily not responding, usually it
91    means there's nothing listening at that address.
92
93    Returned by driver probe() methods to indicate that they
94    found no device to bind to. (ENODEV may also be used.)
95
96EOPNOTSUPP
97    Returned by an adapter when asked to perform an operation
98    that it doesn't, or can't, support.
99
100    For example, this would be returned when an adapter that
101    doesn't support SMBus block transfers is asked to execute
102    one. In that case, the driver making that request should
103    have verified that functionality was supported before it
104    made that block transfer request.
105
106    Similarly, if an I2C adapter can't execute all legal I2C
107    messages, it should return this when asked to perform a
108    transaction it can't. (These limitations can't be seen in
109    the adapter's functionality mask, since the assumption is
110    that if an adapter supports I2C it supports all of I2C.)
111
112EPROTO
113    Returned when slave does not conform to the relevant I2C
114    or SMBus (or chip-specific) protocol specifications. One
115    case is when the length of an SMBus block data response
116    (from the SMBus slave) is outside the range 1-32 bytes.
117
118ETIMEDOUT
119    This is returned by drivers when an operation took too much
120    time, and was aborted before it completed.
121
122    SMBus adapters may return it when an operation took more
123    time than allowed by the SMBus specification; for example,
124    when a slave stretches clocks too far. I2C has no such
125    timeouts, but it's normal for I2C adapters to impose some
126    arbitrary limits (much longer than SMBus!) too.
127
128

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