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 ---------------