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David Brownell81fded12008-07-14 22:38:22 +02001This 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