Documentation/i2c/fault-codes

   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         One example would be a driver trying an SMBus Block Write
  68         with block size outside the range of 1-32 bytes.
  69
  70 EIO
  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
  75 ENODEV
  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
  83 ENOMEM
  84         Returned by any component that can't allocate memory when
  85         it needs to do so.
  86
  87 ENXIO
  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
  96 EOPNOTSUPP
  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
 112 EPROTO
 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
 118 ETIMEDOUT
 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