# 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.

## New DIFs

Developers should use the util/make_new_dif.py script to instantiate some DIF-related templates which follow the guidance below. The script will create:

These files will need checking and editing, but the templates serve to avoid most of the copy/paste required.

Further documentation for the script is provided in the script’s source.

## 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 API Guidance

The following rules specify the basic API that each DIF must conform to. These rules specify the names of types, constants, and functions that each DIF must define for providing certain kinds of non-device-specific functionality (such as initializing handles or managing interrupts).

Notational caveats:

• The token <ip> is the “short IP name” of the peripheral, in snake_case or PascalCase as is appropriate.
• The parameter name handle is not normative, and DIF libraries are free to choose a different, but consistent, name for it.
• All functions below are assumed to return dif_<ip>_result_t or dif_<ip>_<operation>_result_t, so their return values are specified as result_t.
• Unless otherwise noted, all symbols mentioned below are required.

#### Hardware Parameterization

Our aim is that a single DIF library can be used with multiple instances of the same IP on the same chip, even when those IPs have been instantiated with different hardware parameters.

At the moment, we have a good approach to being able to address separate hardware instances instantiated at separate addresses, as long as they have the same hardware parameters (see the base_addr member in dif_<ip>_params_t). Most other parameters come from the specific IP on a case-by-case basis.

As much as possible, we would like to hide these parameters underneath the interface of the DIF, but this is not always possible, especially where particular functionality requires the DIF caller to allocate memory. This should be done even if the current DIF implementation does not support changing parameters, so we can add this parameterization later.

In order to support exposing these parameters to callers, DIFs should provide query functions which take a dif_<ip>_params_t, rather than #defines or global variable definitions. These functions do not have to return dif_<ip>_result_t if they do not error.

An example of such a query function is provided in the template, called dif_<ip>_get_size, but we are not placing restrictions on the naming of these functions.

One implication of this decision is that we will not always be able to provide struct definitions containing fixed-size buffers, which we have relied upon in the past. These structs should instead use a pointer and a size member to safely store the buffer outside the struct and use it without overflows. From the DIF’s perspective, these buffers are dynamically allocated, even if we get static information about their size from other information (e.g. topgen).

#### Base Types

The following basic types are expected to be provided by all DIFs (unless otherwise specified).

• dif_<ip>_t is a type representing a handle to the peripheral. Its fields are private implementation details, and should not be read or written to by clients. This type is usually passed by const pointer, except when it is being initialized (see dif_<ip>_init() below).

• dif_<ip>_params_t is a struct representing hardware instantiation parameters that a DIF library cannot know in advance. Its first field is always the base address for the peripheral registers, styled mmio_region_t base_addr;. This type is always passed by value.

• dif_<ip>_config_t is a struct representing runtime configuration parameters. It is only present when dif_<ip>_configure() is defined. This type is always passed by value.

• dif_<ip>_result_t is an enum representing general return codes for DIFs. It must define the following constants, in 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_<ip>_<operation>_result_t is an enum representing more specific failure modes. These specific return code enums can be shared between multiple DIFs that fail in the same way. <operation> need not correspond to a DIF name if more than one DIF uses these return codes.

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.

• dif_<ip>_toggle_t is an enum representing an enabled or disabled state. This must define exactly two constants, in no particular order:

• kDif<ip>ToggleEnable, indicating an enabled state.
• kDif<ip>ToggleDisable, indicating an enabled state.

This type is intended to be used as a better-named replacement for bool, for operations which are setting wither a behavior is enabled or disabled (such as whether an interrupt is maskable). Values of this type should not be used as if-statement conditions.

If no function requires this type, it may be omitted.

#### Lifecycle Functions

The following functions are the basic functionality for initializing and handling the lifetime of a handle.

• result_t dif_<ip>_init(dif_<ip>_params_t params, dif_<ip>_t *handle); initializes handle in an implementation-defined way, but does not perform any hardware operations. handle may point to uninitialized data on function entry, but if the function returns Ok, then handle must point to initialized data.

• result_t dif_<ip>_configure(const dif_<ip>_t *handle, dif_<ip>_config_t config); configures the hardware managed by handle with runtime parameters in an implementation-defined way. This function should be “one-off”: it should only need to be called once for the lifetime of the handle.

If there is no meaningful state to configure, this function may be omitted. In particular, DIF libraries providing transaction functions will usually have no need for this function at all.

#### Transaction Management

The following types and functions are the standard interface for transaction-oriented peripherals, in which a client schedules an operation to be completed at some point in the future.

• dif_<ip>_transaction_t is a struct representing runtime parameters for starting a hardware transaction. It is only present when dif_<ip>_start() is defined. This type is always passed by value. A DIF library my opt to use another pre-existing type instead, when that type provides a more semanticly appropriate meaning.
• dif_<ip>_output_t is a struct describing how to output a completed transaction. Often, this will be a type like uint8_t *. The same caveats about a DIF library providing a different type apply here.
• result_t dif_<ip>_start(const dif_<ip>_t *handle, dif_<ip>_transaction_t transaction); starts a transaction on a transaction-oriented peripheral. This function may be called multiple times, but each call should be paired with a dif_<ip>_end() call.
• result_t dif_<ip>_end(const dif_<ip>_t *handle, dif_<ip>_output_t out); completes a transaction started with dif_<ip>_start(), writing its results to a location specified in out.

If a peripheral supports multiple transaction modes with incompatible parameter types, the above names may be duplicated by inserting mode_<mode> after <ip>. For example,

result_t dif_<ip>_mode_<mode>_end(const dif_<ip>_t *handle,
dif_<ip>_mode_<mode>_output_t out);


There is no requirement that _start() and _end() share the same set of <mode>s; for example, there might be a single dif_<ip>_start() but many dif_<ip>_mode_<mode>_end()s. This style of API is prefered over using unions with dif_<ip>_transaction_t and dif_<ip>_output_t.

#### Register Locking

The following functions are the standard interface for peripherals that can lock portions of their software-accessible functionality.

• kDif<ip><operation>Locked is the standard variant name for the result enum of an operation that can be locked out. DIFs which may fail due to lockout, which is software-detectable, should return this value when possible.
• result_t dif_<ip>_lock(const dif_<ip>_t *handle); locks out all portions of the peripheral which can be locked. If a peripheral can be locked-out piecewise, dif_<ip>_lock_<operation>() functions may be provided alonside or in lieu of dif_<ip>_lock().
• result_t dif_<ip>_is_locked(const dif_<ip>_t *handle, bool *is_locked); checks whether the peripheral has been locked out. As with dif_<ip>_lock(), DIF libraries may provide a piecewise version of this API.

#### Interrupt Management

The following types and functions are the standard interface for peripherals that provide a collection of INTR_ENABLE, INTR_STATE, and INTR_TEST registers for interrupt management. A DIF library for a peripheral providing such registers must provide this interface.

If a peripheral is defined with no_auto_intr_regs: true, this exact API is not required even if the INTR_ registers are provided (though DIF libraries are encouraged to follow it where it makes sense).

• dif_<ip>_irq_t is an enum that lists all of the interrupt types for this peripheral. A DIF library may opt to use another pre-existing type instead, when, for example, interrupt types are coupled to a distinct peripheral-specific concept. In that case, all occurrences of dif_<ip>_irq_t below should be replaced with this type.
• result_t dif_<ip>_irq_is_pending(const dif_<ip>_t *handle, dif_<ip>_irq_t irq, bool *is_pending); checks whether a specific interrupt is pending (i.e., if the interrupt has been asserted but not yet cleared).
• result_t dif_<ip>_irq_acknowledge(const dif_<ip>_t *handle, dif_<ip>_irq_t irq); acknowledges that an interrupt has been serviced, marking it as complete by clearing its pending bit. This function does nothing and returns Ok if the interrupt wasn’t pending.
• result_t dif_<ip>_irq_get_enabled(const dif_<ip>_t *handle, dif_<ip>_irq_t irq, const dif_<ip>_toggle_t *state); gets whether an interrupt is enabled (i.e., masked).
• result_t dif_<ip>_irq_set_enabled(const dif_<ip>_t *handle, dif_<ip>_irq_t irq, dif_<ip>_toggle_t state); sets whether a particular interrupt is enabled (i.e., masked).
• result_t dif_<ip>_irq_force(const dif_<ip>_t *handle, dif_<ip>_irq_t irq); forcibly asserts a specific interrupt, causing it to be serviced as if hardware had triggered it.

Additionally, the following types allow for batch save/restore operations on the interrupt enable register:

• dif_<ip>_irq_snapshot_t is a type that encapsulates restorable interrupt state, to be used with the two functions below. This type should be treated as opaque by clients.
• result_t dif_<ip>_irq_disable_all(const dif_<ip>_t *handle, dif_<ip>_irq_snapshot_t *snapshot); disables all interrupts associated with the peripheral, saving them to *snapshot. snapshot may be null, in which case the previous enablement state is not saved.
• result_t dif_<ip>_irq_restore_all(const dif_<ip>_t *handle, const dif_<ip>_irq_snapshot_t *snapshot); restores an interrupt enablement snapshot produced by the above function.

#### Unit Testing

Each DIF has an associated unit test, written in C++. Those tests follow the following conventions:

• The whole file is wrapped in the dif_<ip>_unittest namespace.
• There is a base class for all test fixtures, named <ip>Test, which derives testing::Test and mock_mmio::MmioTest.
• Each function has an associated test fixture, usually named <function>Test, which derives <ip>Test. Multiple similar functions may be grouped under one fixture.

### DIF Style 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 one of the result_t enums described above rather than booleans for reporting errors. If a DIF can either error or instead produce a value, it must return a result_t, and use an out-parameter for returning the produced value.

• 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.