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Coding Style

To maintain consistency within the SCP/MCP software source code a series of rules and guidelines have been created; these form the project's coding style.

Style

Unicode

The source code must use the UTF-8 encoding. Comments, documentation and strings may use non-ASCII characters when required (e.g. Greek letters used for units).

License Headers

All files must begin with a license header of the following form, where first year describes the year the file was first upstreamed, and the second year the year when the file was last updated:

Arm SCP/MCP Software Copyright (c) 2015-2021, Arm Limited and Contributors. All rights reserved.

SPDX-License-Identifier: BSD-3-Clause

Clang Format

To aid in establishing a uniform style across the code-base, this project uses Clang Format. When contributing patches, care should be taken to ensure that code has been formatted according to the rules laid down in the .clang-format file, and that deviations occur only where the tool is unable to format code reasonably.

You can automatically format any staged changes with:

git clang-format

Braces

Conditional statements and iteration statements with a single line or multiple lines of code must be surrounded by braces:

if (condition) {
    function_call(
        long_variable_name_x,
        long_variable_name_y,
        long_variable_name_z);
}

This rule applies also to an if-else chain:

if (condition) {
    function_call(
        long_variable_name_x,
        long_variable_name_y,
        long_variable_name_z);
} else {
    function_call_b();
}

Empty loop statements must use continue:

while (condition) {
  continue;
}

Operators

When using operators like sizeof and alignof, where possible use the value-based version over the type-based version:

int counter;

sizeof(counter); /* Preferred over sizeof(int) */

Operator Precedence

Do not rely on the implicit precedence and associativity of operators. Use parentheses to make precedence and associativity explicit:

if ((a == 'a') || (x == 'x')) {
    do_something();
}

Parentheses around a unary operator and its operand may be omitted:

if (!a && *b) {
    do_something();
}

Conventions

Header Guards

All headers must be wrapped with include guards to prevent accidental multiple definitions of header contents. The definition name should be the upper-case file name followed by _H. An example for a file named fwk_mm.h follows:

#ifndef FWK_MM_H
#define FWK_MM_H

(...)

#endif /* FWK_MM_H */

Inclusion Policy

The closing endif statement should be followed directly by a single-line comment which replicates the full guard name. In long files this helps to clarify what is being closed.

Public headers must be included directly except in the following situations:

  • The header is already included by the counterpart to the current C file (e.g. x/src/mod_x.c does not need to include <stdint.h> if x/include/mod_x.h includes it)
  • The header is an internal header and is included as part of another public header (e.g. x/src/mod_x.c does not need to include x/include/internal/x.h if another header includes it)

These rules do not apply to private headers (headers under a src/ directory).

Macros and Constants

All macro and constant names must be written in upper-case to differentiate them from functions and non-constants.

Logical groupings of constants should be defined as enumerations with a common prefix, so that they can be used as parameter types and in constant expressions. Enumerations that contain a "number of elements" value must use the _COUNT suffix, and the final enumeration value should include a comma.

enum command_id {
    COMMAND_ID_VERSION,
    COMMAND_ID_PING,
    COMMAND_ID_EXIT,
    COMMAND_ID_COUNT,
};

void process_cmd(enum command_id id)
{
    (...)
}

Symbol Naming

Function, variable, file name and type names must:

  • Be written in lower-case
  • Have compound words separated by underline characters (_)
  • Have descriptive names, avoiding contractions where possible (e.g. cluster_count instead of clus_cnt)

Avoid using:

  • Camel case syntax (e.g. cssClusterCount)
  • Hungarian notation, encoding types within names (e.g. int iSize)

The following prefixes are reserved for these components:

  • Architecture: arch_
  • Framework: fwk_
  • Module: mod_
    • Module description: module_
    • Module configuration: config_
  • Firmware: fmw_

Assertions

Use fwk_assert() to mark invariants. Invariants are logical assertions about the behaviour of the system which must hold true for the program to be valid. Invariants are usually easy to communicate to human readers, but not very easy to communicate to the compiler.

Use fwk_expect() to mark expectations. Expectations may fail, and they may or may not be handled, but they are not invariants in that they may reasonably occur at some point in time.

Loop Indices

It is acceptable to use the following common placeholder names for loop indices:

  • i
  • j
  • k

Types

Avoid defining custom types with the typedef keyword where possible. Names defined with typedef must use the _t suffix.

Comments

To ensure a consistent look, the preferred style for single-line comments is to use the C89 style of paired forward-slashes and asterisks, ending with no punctuation:

/* A short, single-line comment */

Multi-line comments should follow a similar style, and always end with punctuation:

/*
 * This is a very, very, very, very long multi-line comment where each line
 * starts with an asterisk and paragraphs ends with punctation.
 */

Doxygen comments follow only the second style, for consistency in the generated documentation:

/*!
 * \brief Hello, World.
 */

Preprocessor if 0 is preferred for commenting out blocks of code where it is necessary to do so, as these can be nested (unlike multi-line comment blocks).

void function_a(int x, int y)
{
    (...)
}

#if 0
void function_b(int x, int y)
{
    (...)
}
#endif

Prefer functions to function-like macros where possible. Avoid using the inline keyword, as inlining sites are better-determined by the compiler.

Initialization

When literals require explicit literal initialization, avoid implicit casts by using the proper integer literal suffix or initializer:

void *p = NULL;
unsigned int u = 0u;
float f = 0.0f;
double d = 0.0;
char c = '\0';

Array and structure initialization should use designated initializers. These allow elements to be initialized using array indices or structure field names without a fixed ordering:

uint32_t clock_frequencies[3] = {
    [2] = 800,
    [0] = 675,
};

struct clock clock_cpu = {
    .name = "CPU",
    .frequency = clock_frequencies[2],
};

Device Register Definitions

The format for structures representing memory-mapped device registers is standardized:

  • Register structure names are suffixed with _reg
  • All register names must be capitalized
  • Non-reserved registers must be one of FWK_R, FWK_W, or FWK_RW
  • Bit-field masks and shifts use the <DEVICE>_<REGISTER>_<FIELD> name, suffixed with either _MASK or _POS
#include <stdint.h>
#include <fwk_macros.h>

struct my_timer_reg {
    FWK_RW uint32_t CONFIG;
           uint32_t RESERVED1;
    FWK_W  uint32_t IRQ_CLEAR;
    FWK_R  uint32_t IRQ_STATUS;
           uint8_t  RESERVED2[0x40];
};

#define MY_TIMER_CONFIG_ENABLE UINT32_C(0x00000001)
#define MY_TIMER_CONFIG_SLEEP UINT32_C(0x00000002)

#define MY_TIMER_IRQ_STATUS_ALERT UINT32_C(0x00000001)

Note: A template file can be found in doc/template/device.h

Doxygen Comments

The project APIs are documented using Doxygen comments.

It is mandatory to document every API exposed to other elements of the project. By default, the provided Doxygen configuration omits undocumented elements from the compiled documentation.

At a minimum:

  • All functions and structures must have at least a brief tag
  • All functions must document their parameters (if any) with the param tag
    • Input parameters must use param[in]
    • Output parameters must use param[out]
    • Input/output parameters must use param[in, out]
  • All functions returning a value should use the return and, optionally, the retval tag to document their return value
    • return should document what the type of the return value represents
    • retval should document what individual return values represent

Alignment and indentation:

  • Documentation must also obey the 80 column limit
  • Multiple lines of documentation on an entry (e.g. details) must be indented using the equivalent of two 4-space based tabs from the first column (see example below)

Function documentation example:

/*!
 * \brief Enable the watchdog.
 *
 * \details This function enables the watchdog. If mod_wdog_set_interval() has
 *      not been called beforehand then the watchdog defaults to a 500
 *      millisecond timeout period.
 */
void mod_wdog_enable(void);

Structure documentation example:

/*!
 * \brief A structure example.
 *
 * \details This is an example of a structure for the code style documentation.
 */
struct mod_structure {
    /*!
     * \brief Example field.
     */
    unsigned int example;
};

Python

Python based tools must follow the PEP8 specification. This is enforced with the pycodestyle tool.

Python code must be linted with pylint.