On 3/24/21 5:26 PM, Simon Glass wrote:
Driver model uses quite strong conventions on error codes, but these are currently not clearly documented. Add a description of the commonly used errors.
Signed-off-by: Simon Glass sjg@chromium.org
Changes in v3:
- Add updates based on feedback from Sean Anderson seanga2@gmail.com
Changes in v2:
Add a patch to document the common error codes
doc/driver-model/design.rst | 133 ++++++++++++++++++++++++++++++++++++ 1 file changed, 133 insertions(+)
diff --git a/doc/driver-model/design.rst b/doc/driver-model/design.rst index 4e5cecbab6a..b0e6337030a 100644 --- a/doc/driver-model/design.rst +++ b/doc/driver-model/design.rst @@ -900,6 +900,139 @@ Some special flags are used to determine whether to remove the device: The dm_remove_devices_flags() function can be used to remove devices based on their driver flags.
+Error codes +-----------
+Driver model tries to use errors codes in a consistent way, as follows:
+-EAGAIN
- Try later, e.g. dependencies not ready
+-EINVAL
- Invalid argument, such as `dev_read_...()` failed or any other
- devicetree-related access. Also used when a driver method is passed an
- argument it considers invalid or does not support.
+-EIO
- Failed to perform an I/O operation. This is used when a local device
- (i.e. part of the SOC) does not work as expected. Use -EREMOTEIO for
- failures to talk to a separate device, e.g. over an I2C or SPI
- channel.
+-ENODEV
- Do not bind the device. This should not be used to indicate an
- error probing the device or for any other purpose, lest driver model get
- confused. Using `-ENODEV` inside a driver method makes no sense, since
- clearly there is a device.
+-ENOENT
- Entry or object not found. This is used when a device, file or directory
- cannot be found (e.g. when looked up by name), It can also indicate a
- missing devicetree subnode.
+-ENOMEM
- Out of memory
+-ENOSPC
- Ran out of space (e.g. in a buffer or limited-size array)
+-ENOSYS
- Function not implemented. This is returned by uclasses where the driver does
- not implement a particular method. It can also be returned by drivers when
- a particular sub-method is not implemented. This is widely checked in the
- wider code base, where a feature may or may not be compiled into U-Boot. It
- indicates that the feature is not available, but this is often just normal
- operation. Please do not use -ENOSUPP. If an incorrect or unknown argument
- is provided to a method (e.g. an unknown clock ID), return -EINVAL.
+-ENXIO
- Couldn't find device/address. This is used when a device or address
- could not be obtained or is not valid. It is often used to indicate a
- different type of problem, if -ENOENT is already used for something else in
- the driver.
+-EPERM
- This is -1 so some older code may use it as a generic error. This indicates
- that an operation is not permitted, e.g. a security violation or policy
- constraint. It is returned internally when binding devices before relocation,
- if the device is not marked for pre-relocation use.
+-EPFNOSUPPORT
- Missing uclass. This is deliberately an uncommon error code so that it can
- easily be distinguished. If you see this very early in U-Boot, it means that
- a device exists with a particular uclass but the uclass does not (mostly
- likely because it is not compiled in). Enable DEBUG in uclass.c or lists.c
- to see which uclass ID or driver is causing the problem.
+-EREMOTEIO
- This indicates an error in talking to a peripheral over a comms link, such
- as I2C or SPI. It might indicate that the device is not present or is not
- responding as expected.
+-ETIMEDOUT
- Hardware access or some other operation has timed out. This is used where
- there is an expected time of response and that was exceeded by enough of
- a margin that there is probably something wrong.
+Less common ones:
+-ECOMM
- Not widely used, but similar to -EREMOTEIO. Can be useful as a secondary
- error to distinguish the problem from -EREMOTEIO.
+-EKEYREJECTED
- Attempt to remove a device which does not match the removal flags. See
- device_remove().
+-EILSEQ
- Devicetree read failure, specifically trying to read a string index which
- does not exist, in a string-listg property
+-ENOEXEC
- Attempt to use a uclass method on a device not in that uclass. This is
- seldom checked at present, since it is generally a programming error and a
- waste of code space. A DEBUG-only check would be useful here.
+-ENODATA
- Devicetree read error, where a property exists but has no data associated
- with it
+-EOVERFLOW
- Devicetree read error, where the property is longer than expected
+-EPROBE_DEFER
- Attempt to remove a non-vital device when the removal flags indicate that
- only vital devices should be removed
+-ERANGE
- Returned by regmap functions when arguments are out of range. This can be
- useful for disinguishing regmap errors from other errors obtained while
- probing devices.
+Drivers should use the same conventions so that things function as expected. +In particular, if a driver fails to probe, or a uclass operation fails, the +error code is the primary way to indicate what actually happened.
+Printing error messages in drivers is discouraged due to code size bloat and +since it can result in messages appearing in normal operation. For example, if +a command tries two different devices and uses whichever one probes correctly, +we don't want an error message displayed, even if the command itself might show +a warning or informational message. Ideally, messages in drivers should only be +displayed when debugging, e.g. by using log_debug() although in extreme cases +log_warning() or log_error() may be used.
+Error messages can be logged using `log_msg_ret()`, so that enabling +`CONFIG_LOG` and `CONFIG_LOG_ERROR_RETURN` shows a trace of error codes returned +through the call stack. That can be a handy way of quickly figuring out where +an error occurred. Get into the habit of return errors with +`return log_msg_ret("here", ret)` instead of just `return ret`. The string +just needs to be long enough to find in a single function, since a log record +stores (and can print with `CONFIG_LOGF_FUNC`) the function where it was +generated.
Data Structures
Reviewed-by: Sean Anderson seanga2@gmail.com