128 lines
5 KiB
Text
128 lines
5 KiB
Text
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This is a summary of the most important conventions for use of fault
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codes in the I2C/SMBus stack.
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A "Fault" is not always an "Error"
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----------------------------------
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Not all fault reports imply errors; "page faults" should be a familiar
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example. Software often retries idempotent operations after transient
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faults. There may be fancier recovery schemes that are appropriate in
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some cases, such as re-initializing (and maybe resetting). After such
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recovery, triggered by a fault report, there is no error.
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In a similar way, sometimes a "fault" code just reports one defined
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result for an operation ... it doesn't indicate that anything is wrong
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at all, just that the outcome wasn't on the "golden path".
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In short, your I2C driver code may need to know these codes in order
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to respond correctly. Other code may need to rely on YOUR code reporting
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the right fault code, so that it can (in turn) behave correctly.
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I2C and SMBus fault codes
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-------------------------
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These are returned as negative numbers from most calls, with zero or
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some positive number indicating a non-fault return. The specific
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numbers associated with these symbols differ between architectures,
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though most Linux systems use <asm-generic/errno*.h> numbering.
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Note that the descriptions here are not exhaustive. There are other
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codes that may be returned, and other cases where these codes should
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be returned. However, drivers should not return other codes for these
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cases (unless the hardware doesn't provide unique fault reports).
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Also, codes returned by adapter probe methods follow rules which are
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specific to their host bus (such as PCI, or the platform bus).
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EAGAIN
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Returned by I2C adapters when they lose arbitration in master
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transmit mode: some other master was transmitting different
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data at the same time.
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Also returned when trying to invoke an I2C operation in an
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atomic context, when some task is already using that I2C bus
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to execute some other operation.
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EBADMSG
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Returned by SMBus logic when an invalid Packet Error Code byte
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is received. This code is a CRC covering all bytes in the
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transaction, and is sent before the terminating STOP. This
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fault is only reported on read transactions; the SMBus slave
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may have a way to report PEC mismatches on writes from the
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host. Note that even if PECs are in use, you should not rely
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on these as the only way to detect incorrect data transfers.
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EBUSY
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Returned by SMBus adapters when the bus was busy for longer
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than allowed. This usually indicates some device (maybe the
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SMBus adapter) needs some fault recovery (such as resetting),
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or that the reset was attempted but failed.
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EINVAL
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This rather vague error means an invalid parameter has been
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detected before any I/O operation was started. Use a more
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specific fault code when you can.
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One example would be a driver trying an SMBus Block Write
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with block size outside the range of 1-32 bytes.
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EIO
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This rather vague error means something went wrong when
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performing an I/O operation. Use a more specific fault
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code when you can.
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ENODEV
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Returned by driver probe() methods. This is a bit more
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specific than ENXIO, implying the problem isn't with the
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address, but with the device found there. Driver probes
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may verify the device returns *correct* responses, and
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return this as appropriate. (The driver core will warn
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about probe faults other than ENXIO and ENODEV.)
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ENOMEM
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Returned by any component that can't allocate memory when
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it needs to do so.
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ENXIO
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Returned by I2C adapters to indicate that the address phase
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of a transfer didn't get an ACK. While it might just mean
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an I2C device was temporarily not responding, usually it
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means there's nothing listening at that address.
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Returned by driver probe() methods to indicate that they
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found no device to bind to. (ENODEV may also be used.)
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EOPNOTSUPP
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Returned by an adapter when asked to perform an operation
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that it doesn't, or can't, support.
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For example, this would be returned when an adapter that
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doesn't support SMBus block transfers is asked to execute
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one. In that case, the driver making that request should
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have verified that functionality was supported before it
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made that block transfer request.
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Similarly, if an I2C adapter can't execute all legal I2C
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messages, it should return this when asked to perform a
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transaction it can't. (These limitations can't be seen in
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the adapter's functionality mask, since the assumption is
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that if an adapter supports I2C it supports all of I2C.)
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EPROTO
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Returned when slave does not conform to the relevant I2C
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or SMBus (or chip-specific) protocol specifications. One
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case is when the length of an SMBus block data response
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(from the SMBus slave) is outside the range 1-32 bytes.
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ETIMEDOUT
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This is returned by drivers when an operation took too much
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time, and was aborted before it completed.
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SMBus adapters may return it when an operation took more
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time than allowed by the SMBus specification; for example,
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when a slave stretches clocks too far. I2C has no such
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timeouts, but it's normal for I2C adapters to impose some
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arbitrary limits (much longer than SMBus!) too.
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