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.
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:
- A Header for the DIF, based on
- A Checklist for the DIF, based on
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.
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.
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
- 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).
- The token
<ip>is the “short IP name” of the peripheral, in
PascalCaseas is appropriate.
- The parameter name
handleis 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>_<operation>_result_t, so their return values are specified as
- Unless otherwise noted, all symbols mentioned below are required.
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
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
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
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).
The following basic types are expected to be provided by all DIFs (unless otherwise specified).
dif_<ip>_tis 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
constpointer, except when it is being initialized (see
dif_<ip>_params_tis 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_tis 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_tis 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>BadArgcannot 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_tis 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><operation>Error, with value
kDif<ip><operation>BadArg, with value
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_tis 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 no function requires this type, it may be omitted.
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
handlein an implementation-defined way, but does not perform any hardware operations.
handlemay point to uninitialized data on function entry, but if the function returns
handlemust point to initialized data.
result_t dif_<ip>_configure(const dif_<ip>_t *handle, dif_<ip>_config_t config);configures the hardware managed by
handlewith 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.
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_tis 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_tis 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
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
If a peripheral supports multiple transaction modes with incompatible parameter
types, the above names may be duplicated by inserting
result_t dif_<ip>_mode_<mode>_end(const dif_<ip>_t *handle, dif_<ip>_mode_<mode>_output_t out);
There is no requirement that
_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
The following functions are the standard interface for peripherals that can lock portions of their software-accessible functionality.
kDif<ip><operation>Lockedis 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
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.
The following types and functions are the standard interface for peripherals that
provide a collection of
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_tis 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_tbelow 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
Okif 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_tis 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
snapshotmay 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.
Each DIF has an associated unit test, written in C++. Those tests follow the following conventions:
- The whole file is wrapped in the
- There is a base class for all test fixtures, named
<ip>Test, which derives
- 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
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
volatile(cv-qualified) in DIF signatures.
- Scalar arguments must not be declared
DIFs must use one of the
result_tenums 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.
- If a DIF returns
- 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
statichelper functions that return an error of some kind as well.
- DIFs that cannot error and that do not return a value must return
- 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.
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_tare 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.
- DIFs must ensure their pointer arguments are non-null, unless that pointer is for an optional out-parameter. Arguments typed
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>Erroror 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
- Enum switches do not need a
casefor enum constants that are unreachable due to a guard statement.
- 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
DIFs must use
sw/device/lib/base/mmio.hfor accessing memory-mapped hardware. DIFs must not use
sw/device/lib/base/memory.hfor 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
staticDIF functions should not be marked
- An internal DIF function does not need to check preconditions, if all the DIF functions that call it have already checked that precondition.