# The OpenTitan DIF Library

A DIF is a “Device Interface Function”. DIFs are low-level routines for accessing the hardware functionality directly, and are agnostic to the particular environment or context they are called from. The intention is that DIFs can be used during design verification, and during early silicon verification, and by the high-level driver software in production firmware.

This subtree provides headers and libraries known collectively as the DIF libraries.

There is one DIF library per hardware IP, and each one contains the DIFs required to actuate all of the specification-required functionality of the hardware they are written for.

## Checklists

This directory also contains checklists for each DIF, in markdown format. They are linked to from the Hardware Dashboard, in the Development Stage column.

## DIF Style Guide

DIFs are very low-level software, so they have a more rigorous coding style than other parts of the codebase.

DIFs should follow the OpenTitan C/C++ style guide where it does not contradict with the guidelines below.

The guidelines below apply to writing DIFs, and code should be written in a similar style to the existing DIF libraries in this directory.

### Definitions

Side-effects include (but are not limited to) writing to memory, including memory-mapped hardware, and modifying processor CSRs.

### DIF Library Guidance

• DIF libraries must be written in C.
• DIF libraries can only depend on the following headers (and their associated libraries) from the sw/device/lib/base directory:
• sw/device/lib/base/bitfield.h
• sw/device/lib/base/mmio.h
• sw/device/lib/base/memory.h
• DIF libraries must not depend on other DIF libraries. Exercising DIF functionality may require an environment set up using another DIF library, but DIFs must not call DIFs in other DIF libraries.
• DIF library headers must be polyglot headers for C and C++.

### DIF Guidance

The following rules must be followed by public DIF functions (those declared in the DIF library's header file). Internal DIF functions (those declared static and not declared in the DIF library's header file) should follow these rules but there are some relaxations of these rules for them described at the end.

• DIF declarations must match their definitions exactly.

• Scalar arguments must not be declared const or volatile (cv-qualified) in DIF signatures.
• DIFs must use enum return codes rather than booleans for reporting errors. If a DIF can either error or instead produce a value, it must return an enum return code, and use an out-parameter for returning the produced value.

• DIF libraries must have a general return code enum called dif_<ip>_result_t. It must define the following constants (in this order):

• kDif<ip>Ok, with value 0, to denote the call succeeded.
• kDif<ip>Error, with value 1, to denote a non-specific error happened during the call. This is for the default: case of enum switches (as noted below), and for assertion errors (usually where the function has already caused side-effects so kDif<ip>BadArg cannot be used).
• kDif<ip>BadArg, with value 2, to denote that the caller supplied incorrect arguments. This value must only be returned if the function has not caused any side-effects.
• DIF libraries should define specific return code enums for operations that fail in more specific ways. These specific return code enums can be shared between multiple DIFs that fail in the same way.

These more specific return code types must be named dif_<ip>_<operation>_result_t, and their constants must be prefixed with kDif<ip><operation>. <operation> need not correspond to a DIF name if the return codes are shared between DIFs.

The first three constants in these specific enums must define the following constants:

• kDif<ip><operation>Ok, with value kDif<ip>Ok,
• kDif<ip><operation>Error, with value kDif<ip>Error, and
• kDif<ip><operation>BadArg, with value kDif<ip>BadArg.

Additional, specific return code constants must all be defined after these three general constants, and may cover more specific forms of the return codes defined above, including more specific reasons arguments are invalid.

• DIFs must document the meaning of each return code constant, including the required ones above, with a Doxygen comment per declaration. This comment must include whether returning this error code could have left the hardware in an invalid or unrecoverable state.

• If a DIF returns kDif<ip>Error, it must be assumed to have left the hardware in an invalid, unrecoverable state.
• If a DIF returns kDif<ip>BadArg, it must leave the hardware in a valid and recoverable state. This is in addition to the rule that this value may only be returned if the function has not caused any side-effects.
• DIFs that return an enum return code must be annotated with __attribute__((warn_unused_result)), to help minimize mistakes from failing to check a result. This guidance applies to static helper functions that return an error of some kind as well.

• DIFs that cannot error and that do not return a value must return void.

• DIFs must check their arguments against preconditions using “guard statements”. A guard statement is a simple if statement at the start of a function which only returns an error code if the preconditions are not met. Guard statements must cover the following checks:

• DIFs must ensure their pointer arguments are non-null, unless that pointer is for an optional out-parameter. Arguments typed mmio_region_t are not pointers, and cannot meaningfully be checked for non-nullness.
• DIFs must ensure, if they only accept a subset of an enum, that the argument is within that subset. However, DIFs may assume, for checking preconditions, that any enum argument is one of the enum constants.
• DIFs must not cause any side-effects before any guard statements. This means returning early from a guard statement must not leave the hardware in an invalid or unrecoverable state.
• Switch statements in DIFs must always have a default case, including when switching on an enum value (an “enum switch”).

• The default case of an enum switch must report an error for values that are not a constant from that enum. In the absence of more specific information, this should return kDif<ip>Error or the equivalent return code value from a more specific return code enum. If the enum switch is part of a guard statement, it may return kDif<ip>BadArg instead.
• Enum switches do not need a case for enum constants that are unreachable due to a guard statement.
• DIFs must use sw/device/lib/base/mmio.h for accessing memory-mapped hardware. DIFs must not use sw/device/lib/base/memory.h for accessing memory-mapped hardware.

• Internal DIF functions, which are not intended to be part of a public DIF library interface, must not be declared in the DIF library header, and must be marked static.

• static DIF functions should not be marked static inline.
• An internal DIF function does not need to check preconditions, if all the DIF functions that call it have already checked that precondition.