GPIO Character Device Userspace API¶

This is latest version (v2) of the character device API, as defined in include/uapi/linux/gpio.h.

First added in 5.10.

Note

Do NOT abuse userspace APIs to control hardware that has proper kernel drivers. There may already be a driver for your use case, and an existing kernel driver is sure to provide a superior solution to bitbashing from userspace.

Read Subsystem drivers using GPIO to avoid reinventing kernel wheels in userspace.

Similarly, for multi-function lines there may be other subsystems, such as Serial Peripheral Interface (SPI), I2C/SMBus Subsystem, Pulse Width Modulation (PWM) interface, 1-Wire Subsystem etc, that provide suitable drivers and APIs for your hardware.

Basic examples using the character device API can be found in tools/gpio/*.

The API is based around two major objects, the Chip and the Line Request.

Chip¶

The Chip represents a single GPIO chip and is exposed to userspace using device files of the form /dev/gpiochipX.

Each chip supports a number of GPIO lines, chip.lines. Lines on the chip are identified by an offset in the range from 0 to chip.lines - 1, i.e. [0,chip.lines).

Lines are requested from the chip using GPIO_V2_GET_LINE_IOCTL and the resulting line request is used to access the GPIO chip’s lines or monitor the lines for edge events.

Within this documentation, the file descriptor returned by calling open() on the GPIO device file is referred to as chip_fd.

Operations¶

The following operations may be performed on the chip:

Line Request¶

Line requests are created by GPIO_V2_GET_LINE_IOCTL and provide access to a set of requested lines. The line request is exposed to userspace via the anonymous file descriptor returned in request.fd by GPIO_V2_GET_LINE_IOCTL.

Within this documentation, the line request file descriptor is referred to as req_fd.

Operations¶

The following operations may be performed on the line request:

Types¶

This section contains the structs and enums that are referenced by the API v2, as defined in include/uapi/linux/gpio.h.

struct gpiochip_info¶: Information about a certain GPIO chip

Definition:

struct gpiochip_info {
    char name[GPIO_MAX_NAME_SIZE];
    char label[GPIO_MAX_NAME_SIZE];
    __u32 lines;
};

Members

name: the Linux kernel name of this GPIO chip
label: a functional name for this GPIO chip, such as a product number, may be empty (i.e. label[0] == ‘0’)
lines: number of GPIO lines on this chip

enum gpio_v2_line_flag¶: struct gpio_v2_line_attribute.flags values

Constants

GPIO_V2_LINE_FLAG_USED: line is not available for request
GPIO_V2_LINE_FLAG_ACTIVE_LOW: line active state is physical low
GPIO_V2_LINE_FLAG_INPUT: line is an input
GPIO_V2_LINE_FLAG_OUTPUT: line is an output
GPIO_V2_LINE_FLAG_EDGE_RISING: line detects rising (inactive to active) edges
GPIO_V2_LINE_FLAG_EDGE_FALLING: line detects falling (active to inactive) edges
GPIO_V2_LINE_FLAG_OPEN_DRAIN: line is an open drain output
GPIO_V2_LINE_FLAG_OPEN_SOURCE: line is an open source output
GPIO_V2_LINE_FLAG_BIAS_PULL_UP: line has pull-up bias enabled
GPIO_V2_LINE_FLAG_BIAS_PULL_DOWN: line has pull-down bias enabled
GPIO_V2_LINE_FLAG_BIAS_DISABLED: line has bias disabled
GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME: line events contain REALTIME timestamps
GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE: line events contain timestamps from the hardware timestamping engine (HTE) subsystem

struct gpio_v2_line_values¶: Values of GPIO lines

Definition:

struct gpio_v2_line_values {
    __aligned_u64 bits;
    __aligned_u64 mask;
};

Members

bits: a bitmap containing the value of the lines, set to 1 for active and 0 for inactive
mask: a bitmap identifying the lines to get or set, with each bit number corresponding to the index into struct gpio_v2_line_request.offsets

enum gpio_v2_line_attr_id¶: struct gpio_v2_line_attribute.id values identifying which field of the attribute union is in use.

Constants

GPIO_V2_LINE_ATTR_ID_FLAGS: flags field is in use
GPIO_V2_LINE_ATTR_ID_OUTPUT_VALUES: values field is in use
GPIO_V2_LINE_ATTR_ID_DEBOUNCE: debounce_period_us field is in use

struct gpio_v2_line_attribute¶: a configurable attribute of a line

Definition:

struct gpio_v2_line_attribute {
    __u32 id;
    __u32 padding;
    union {
        __aligned_u64 flags;
        __aligned_u64 values;
        __u32 debounce_period_us;
    };
};

Members

id: attribute identifier with value from enum gpio_v2_line_attr_id
padding: reserved for future use and must be zero filled
{unnamed_union}: anonymous
flags: if id is GPIO_V2_LINE_ATTR_ID_FLAGS, the flags for the GPIO line, with values from enum gpio_v2_line_flag, such as GPIO_V2_LINE_FLAG_ACTIVE_LOW, GPIO_V2_LINE_FLAG_OUTPUT etc, added together. This overrides the default flags contained in the struct gpio_v2_line_config for the associated line.
values: if id is GPIO_V2_LINE_ATTR_ID_OUTPUT_VALUES, a bitmap containing the values to which the lines will be set, with each bit number corresponding to the index into struct gpio_v2_line_request.offsets
debounce_period_us: if id is GPIO_V2_LINE_ATTR_ID_DEBOUNCE, the desired debounce period, in microseconds

struct gpio_v2_line_config_attribute¶: a configuration attribute associated with one or more of the requested lines.

Definition:

struct gpio_v2_line_config_attribute {
    struct gpio_v2_line_attribute attr;
    __aligned_u64 mask;
};

Members

attr: the configurable attribute
mask: a bitmap identifying the lines to which the attribute applies, with each bit number corresponding to the index into struct gpio_v2_line_request.offsets

struct gpio_v2_line_config¶: Configuration for GPIO lines

Definition:

struct gpio_v2_line_config {
    __aligned_u64 flags;
    __u32 num_attrs;
    __u32 padding[5];
    struct gpio_v2_line_config_attribute attrs[GPIO_V2_LINE_NUM_ATTRS_MAX];
};

Members

flags: flags for the GPIO lines, with values from enum gpio_v2_line_flag, such as GPIO_V2_LINE_FLAG_ACTIVE_LOW, GPIO_V2_LINE_FLAG_OUTPUT etc, added together. This is the default for all requested lines but may be overridden for particular lines using attrs.
num_attrs: the number of attributes in attrs
padding: reserved for future use and must be zero filled
attrs: the configuration attributes associated with the requested lines. Any attribute should only be associated with a particular line once. If an attribute is associated with a line multiple times then the first occurrence (i.e. lowest index) has precedence.

struct gpio_v2_line_request¶: Information about a request for GPIO lines

Definition:

struct gpio_v2_line_request {
    __u32 offsets[GPIO_V2_LINES_MAX];
    char consumer[GPIO_MAX_NAME_SIZE];
    struct gpio_v2_line_config config;
    __u32 num_lines;
    __u32 event_buffer_size;
    __u32 padding[5];
    __s32 fd;
};

Members

offsets: an array of desired lines, specified by offset index for the associated GPIO chip
consumer: a desired consumer label for the selected GPIO lines such as “my-bitbanged-relay”
config: requested configuration for the lines
num_lines: number of lines requested in this request, i.e. the number of valid fields in the GPIO_V2_LINES_MAX sized arrays, set to 1 to request a single line
event_buffer_size: a suggested minimum number of line events that the kernel should buffer. This is only relevant if edge detection is enabled in the configuration. Note that this is only a suggested value and the kernel may allocate a larger buffer or cap the size of the buffer. If this field is zero then the buffer size defaults to a minimum of num_lines * 16.
padding: reserved for future use and must be zero filled
fd: after a successful GPIO_V2_GET_LINE_IOCTL operation, contains a valid anonymous file descriptor representing the request

struct gpio_v2_line_info¶: Information about a certain GPIO line

Definition:

struct gpio_v2_line_info {
    char name[GPIO_MAX_NAME_SIZE];
    char consumer[GPIO_MAX_NAME_SIZE];
    __u32 offset;
    __u32 num_attrs;
    __aligned_u64 flags;
    struct gpio_v2_line_attribute attrs[GPIO_V2_LINE_NUM_ATTRS_MAX];
    __u32 padding[4];
};

Members

name: the name of this GPIO line, such as the output pin of the line on the chip, a rail or a pin header name on a board, as specified by the GPIO chip, may be empty (i.e. name[0] == ‘0’)
consumer: a functional name for the consumer of this GPIO line as set by whatever is using it, will be empty if there is no current user but may also be empty if the consumer doesn’t set this up
offset: the local offset on this GPIO chip, fill this in when requesting the line information from the kernel
num_attrs: the number of attributes in attrs
flags: flags for this GPIO line, with values from enum gpio_v2_line_flag, such as GPIO_V2_LINE_FLAG_ACTIVE_LOW, GPIO_V2_LINE_FLAG_OUTPUT etc, added together
attrs: the configuration attributes associated with the line
padding: reserved for future use

enum gpio_v2_line_changed_type¶: struct gpio_v2_line_changed.event_type values

Constants

GPIO_V2_LINE_CHANGED_REQUESTED: line has been requested
GPIO_V2_LINE_CHANGED_RELEASED: line has been released
GPIO_V2_LINE_CHANGED_CONFIG: line has been reconfigured

struct gpio_v2_line_info_changed¶: Information about a change in status of a GPIO line

Definition:

struct gpio_v2_line_info_changed {
    struct gpio_v2_line_info info;
    __aligned_u64 timestamp_ns;
    __u32 event_type;
    __u32 padding[5];
};

Members

info: updated line information
timestamp_ns: estimate of time of status change occurrence, in nanoseconds
event_type: the type of change with a value from enum gpio_v2_line_changed_type
padding: reserved for future use

enum gpio_v2_line_event_id¶: struct gpio_v2_line_event.id values

Constants

GPIO_V2_LINE_EVENT_RISING_EDGE: event triggered by a rising edge
GPIO_V2_LINE_EVENT_FALLING_EDGE: event triggered by a falling edge

struct gpio_v2_line_event¶: The actual event being pushed to userspace

Definition:

struct gpio_v2_line_event {
    __aligned_u64 timestamp_ns;
    __u32 id;
    __u32 offset;
    __u32 seqno;
    __u32 line_seqno;
    __u32 padding[6];
};

Members

timestamp_ns: best estimate of time of event occurrence, in nanoseconds
id: event identifier with value from enum gpio_v2_line_event_id
offset: the offset of the line that triggered the event
seqno: the sequence number for this event in the sequence of events for all the lines in this line request
line_seqno: the sequence number for this event in the sequence of events on this particular line
padding: reserved for future use

Description

By default the timestamp_ns is read from CLOCK_MONOTONIC and is intended to allow the accurate measurement of the time between events. It does not provide the wall-clock time.

If the GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME flag is set then the timestamp_ns is read from CLOCK_REALTIME.

If the GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE flag is set then the timestamp_ns is provided by the hardware timestamping engine (HTE) subsystem.

The Linux Kernel

Contents

This Page

GPIO Character Device Userspace API¶

Chip¶

Operations¶

Line Request¶

Operations¶

Types¶