Linux USB HID gadget driver — The Linux Kernel documentation (2024)

Introduction

The HID Gadget driver provides emulation of USB Human InterfaceDevices (HID). The basic HID handling is done in the kernel,and HID reports can be sent/received through I/O on the/dev/hidgX character devices.

For more details about HID, see the developer page onhttps://www.usb.org/developers/hidpage/

Configuration

g_hid is a platform driver, so to use it you need to addstruct platform_device(s) to your platform code defining theHID function descriptors you want to use - E.G. somethinglike:

#include <linux/platform_device.h>#include <linux/usb/g_hid.h>/* hid descriptor for a keyboard */static struct hidg_func_descriptor my_hid_data = { .subclass = 0, /* No subclass */ .protocol = 1, /* Keyboard */ .report_length = 8, .report_desc_length = 63, .report_desc = { 0x05, 0x01, /* USAGE_PAGE (Generic Desktop) */ 0x09, 0x06, /* USAGE (Keyboard) */ 0xa1, 0x01, /* COLLECTION (Application) */ 0x05, 0x07, /* USAGE_PAGE (Keyboard) */ 0x19, 0xe0, /* USAGE_MINIMUM (Keyboard LeftControl) */ 0x29, 0xe7, /* USAGE_MAXIMUM (Keyboard Right GUI) */ 0x15, 0x00, /* LOGICAL_MINIMUM (0) */ 0x25, 0x01, /* LOGICAL_MAXIMUM (1) */ 0x75, 0x01, /* REPORT_SIZE (1) */ 0x95, 0x08, /* REPORT_COUNT (8) */ 0x81, 0x02, /* INPUT (Data,Var,Abs) */ 0x95, 0x01, /* REPORT_COUNT (1) */ 0x75, 0x08, /* REPORT_SIZE (8) */ 0x81, 0x03, /* INPUT (Cnst,Var,Abs) */ 0x95, 0x05, /* REPORT_COUNT (5) */ 0x75, 0x01, /* REPORT_SIZE (1) */ 0x05, 0x08, /* USAGE_PAGE (LEDs) */ 0x19, 0x01, /* USAGE_MINIMUM (Num Lock) */ 0x29, 0x05, /* USAGE_MAXIMUM (Kana) */ 0x91, 0x02, /* OUTPUT (Data,Var,Abs) */ 0x95, 0x01, /* REPORT_COUNT (1) */ 0x75, 0x03, /* REPORT_SIZE (3) */ 0x91, 0x03, /* OUTPUT (Cnst,Var,Abs) */ 0x95, 0x06, /* REPORT_COUNT (6) */ 0x75, 0x08, /* REPORT_SIZE (8) */ 0x15, 0x00, /* LOGICAL_MINIMUM (0) */ 0x25, 0x65, /* LOGICAL_MAXIMUM (101) */ 0x05, 0x07, /* USAGE_PAGE (Keyboard) */ 0x19, 0x00, /* USAGE_MINIMUM (Reserved) */ 0x29, 0x65, /* USAGE_MAXIMUM (Keyboard Application) */ 0x81, 0x00, /* INPUT (Data,Ary,Abs) */ 0xc0 /* END_COLLECTION */ }};static struct platform_device my_hid = { .name = "hidg", .id = 0, .num_resources = 0, .resource = 0, .dev.platform_data = &my_hid_data,};

You can add as many HID functions as you want, only limited bythe amount of interrupt endpoints your gadget driver supports.

Configuration with configfs

Instead of adding fake platform devices and drivers in order to passsome data to the kernel, if HID is a part of a gadget composed withconfigfs the hidg_func_descriptor.report_desc is passed to the kernelby writing the appropriate stream of bytes to a configfs attribute.

Send and receive HID reports

HID reports can be sent/received using read/write on the/dev/hidgX character devices. See below for an example programto do this.

hid_gadget_test is a small interactive program to test the HIDgadget driver. To use, point it at a hidg device and set thedevice type (keyboard / mouse / joystick) - E.G.:

# hid_gadget_test /dev/hidg0 keyboard

You are now in the prompt of hid_gadget_test. You can type anycombination of options and values. Available options andvalues are listed at program start. In keyboard mode you cansend up to six values.

For example type: g i s t r --left-shift

Hit return and the corresponding report will be sent by theHID gadget.

Another interesting example is the caps lock test. Type--caps-lock and hit return. A report is then sent by thegadget and you should receive the host answer, correspondingto the caps lock LED status:

--caps-lockrecv report:2

With this command:

# hid_gadget_test /dev/hidg1 mouse

You can test the mouse emulation. Values are two signed numbers.

Sample code:

 /* hid_gadget_test */ #include <pthread.h> #include <string.h> #include <stdio.h> #include <ctype.h> #include <fcntl.h> #include <errno.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #define BUF_LEN 512 struct options { const char *opt; unsigned char val;};static struct options kmod[] = { {.opt = "--left-ctrl", .val = 0x01}, {.opt = "--right-ctrl", .val = 0x10}, {.opt = "--left-shift", .val = 0x02}, {.opt = "--right-shift", .val = 0x20}, {.opt = "--left-alt", .val = 0x04}, {.opt = "--right-alt", .val = 0x40}, {.opt = "--left-meta", .val = 0x08}, {.opt = "--right-meta", .val = 0x80}, {.opt = NULL}};static struct options kval[] = { {.opt = "--return", .val = 0x28}, {.opt = "--esc", .val = 0x29}, {.opt = "--bckspc", .val = 0x2a}, {.opt = "--tab", .val = 0x2b}, {.opt = "--spacebar", .val = 0x2c}, {.opt = "--caps-lock", .val = 0x39}, {.opt = "--f1", .val = 0x3a}, {.opt = "--f2", .val = 0x3b}, {.opt = "--f3", .val = 0x3c}, {.opt = "--f4", .val = 0x3d}, {.opt = "--f5", .val = 0x3e}, {.opt = "--f6", .val = 0x3f}, {.opt = "--f7", .val = 0x40}, {.opt = "--f8", .val = 0x41}, {.opt = "--f9", .val = 0x42}, {.opt = "--f10", .val = 0x43}, {.opt = "--f11", .val = 0x44}, {.opt = "--f12", .val = 0x45}, {.opt = "--insert", .val = 0x49}, {.opt = "--home", .val = 0x4a}, {.opt = "--pageup", .val = 0x4b}, {.opt = "--del", .val = 0x4c}, {.opt = "--end", .val = 0x4d}, {.opt = "--pagedown", .val = 0x4e}, {.opt = "--right", .val = 0x4f}, {.opt = "--left", .val = 0x50}, {.opt = "--down", .val = 0x51}, {.opt = "--kp-enter", .val = 0x58}, {.opt = "--up", .val = 0x52}, {.opt = "--num-lock", .val = 0x53}, {.opt = NULL}};int keyboard_fill_report(char report[8], char buf[BUF_LEN], int *hold){ char *tok = strtok(buf, " "); int key = 0; int i = 0; for (; tok != NULL; tok = strtok(NULL, " ")) { if (strcmp(tok, "--quit") == 0) return -1; if (strcmp(tok, "--hold") == 0) { *hold = 1; continue; } if (key < 6) { for (i = 0; kval[i].opt != NULL; i++) if (strcmp(tok, kval[i].opt) == 0) { report[2 + key++] = kval[i].val; break; } if (kval[i].opt != NULL) continue; } if (key < 6) if (islower(tok[0])) { report[2 + key++] = (tok[0] - ('a' - 0x04)); continue; } for (i = 0; kmod[i].opt != NULL; i++) if (strcmp(tok, kmod[i].opt) == 0) { report[0] = report[0] | kmod[i].val; break; } if (kmod[i].opt != NULL) continue; if (key < 6) fprintf(stderr, "unknown option: %s\n", tok); } return 8;}static struct options mmod[] = { {.opt = "--b1", .val = 0x01}, {.opt = "--b2", .val = 0x02}, {.opt = "--b3", .val = 0x04}, {.opt = NULL}};int mouse_fill_report(char report[8], char buf[BUF_LEN], int *hold){ char *tok = strtok(buf, " "); int mvt = 0; int i = 0; for (; tok != NULL; tok = strtok(NULL, " ")) { if (strcmp(tok, "--quit") == 0) return -1; if (strcmp(tok, "--hold") == 0) { *hold = 1; continue; } for (i = 0; mmod[i].opt != NULL; i++) if (strcmp(tok, mmod[i].opt) == 0) { report[0] = report[0] | mmod[i].val; break; } if (mmod[i].opt != NULL) continue; if (!(tok[0] == '-' && tok[1] == '-') && mvt < 2) { errno = 0; report[1 + mvt++] = (char)strtol(tok, NULL, 0); if (errno != 0) { fprintf(stderr, "Bad value:'%s'\n", tok); report[1 + mvt--] = 0; } continue; } fprintf(stderr, "unknown option: %s\n", tok); } return 3;}static struct options jmod[] = { {.opt = "--b1", .val = 0x10}, {.opt = "--b2", .val = 0x20}, {.opt = "--b3", .val = 0x40}, {.opt = "--b4", .val = 0x80}, {.opt = "--hat1", .val = 0x00}, {.opt = "--hat2", .val = 0x01}, {.opt = "--hat3", .val = 0x02}, {.opt = "--hat4", .val = 0x03}, {.opt = "--hatneutral", .val = 0x04}, {.opt = NULL}};int joystick_fill_report(char report[8], char buf[BUF_LEN], int *hold){ char *tok = strtok(buf, " "); int mvt = 0; int i = 0; *hold = 1; /* set default hat position: neutral */ report[3] = 0x04; for (; tok != NULL; tok = strtok(NULL, " ")) { if (strcmp(tok, "--quit") == 0) return -1; for (i = 0; jmod[i].opt != NULL; i++) if (strcmp(tok, jmod[i].opt) == 0) { report[3] = (report[3] & 0xF0) | jmod[i].val; break; } if (jmod[i].opt != NULL) continue; if (!(tok[0] == '-' && tok[1] == '-') && mvt < 3) { errno = 0; report[mvt++] = (char)strtol(tok, NULL, 0); if (errno != 0) { fprintf(stderr, "Bad value:'%s'\n", tok); report[mvt--] = 0; } continue; } fprintf(stderr, "unknown option: %s\n", tok); } return 4;}void print_options(char c){ int i = 0; if (c == 'k') { printf(" keyboard options:\n" " --hold\n"); for (i = 0; kmod[i].opt != NULL; i++) printf("\t\t%s\n", kmod[i].opt); printf("\n keyboard values:\n" " [a-z] or\n"); for (i = 0; kval[i].opt != NULL; i++) printf("\t\t%-8s%s", kval[i].opt, i % 2 ? "\n" : ""); printf("\n"); } else if (c == 'm') { printf(" mouse options:\n" " --hold\n"); for (i = 0; mmod[i].opt != NULL; i++) printf("\t\t%s\n", mmod[i].opt); printf("\n mouse values:\n" " Two signed numbers\n" "--quit to close\n"); } else { printf(" joystick options:\n"); for (i = 0; jmod[i].opt != NULL; i++) printf("\t\t%s\n", jmod[i].opt); printf("\n joystick values:\n" " three signed numbers\n" "--quit to close\n"); }}int main(int argc, const char *argv[]){ const char *filename = NULL; int fd = 0; char buf[BUF_LEN]; int cmd_len; char report[8]; int to_send = 8; int hold = 0; fd_set rfds; int retval, i; if (argc < 3) { fprintf(stderr, "Usage: %s devname mouse|keyboard|joystick\n", argv[0]); return 1; } if (argv[2][0] != 'k' && argv[2][0] != 'm' && argv[2][0] != 'j') return 2; filename = argv[1]; if ((fd = open(filename, O_RDWR, 0666)) == -1) { perror(filename); return 3; } print_options(argv[2][0]); while (42) { FD_ZERO(&rfds); FD_SET(STDIN_FILENO, &rfds); FD_SET(fd, &rfds); retval = select(fd + 1, &rfds, NULL, NULL, NULL); if (retval == -1 && errno == EINTR) continue; if (retval < 0) { perror("select()"); return 4; } if (FD_ISSET(fd, &rfds)) { cmd_len = read(fd, buf, BUF_LEN - 1); printf("recv report:"); for (i = 0; i < cmd_len; i++) printf(" %02x", buf[i]); printf("\n"); } if (FD_ISSET(STDIN_FILENO, &rfds)) { memset(report, 0x0, sizeof(report)); cmd_len = read(STDIN_FILENO, buf, BUF_LEN - 1); if (cmd_len == 0) break; buf[cmd_len - 1] = '\0'; hold = 0; memset(report, 0x0, sizeof(report)); if (argv[2][0] == 'k') to_send = keyboard_fill_report(report, buf, &hold); else if (argv[2][0] == 'm') to_send = mouse_fill_report(report, buf, &hold); else to_send = joystick_fill_report(report, buf, &hold); if (to_send == -1) break; if (write(fd, report, to_send) != to_send) { perror(filename); return 5; } if (!hold) { memset(report, 0x0, sizeof(report)); if (write(fd, report, to_send) != to_send) { perror(filename); return 6; } } } } close(fd); return 0;}
Linux USB HID gadget driver — The Linux Kernel  documentation (2024)

References

Top Articles
Latest Posts
Article information

Author: Pres. Carey Rath

Last Updated:

Views: 6202

Rating: 4 / 5 (61 voted)

Reviews: 92% of readers found this page helpful

Author information

Name: Pres. Carey Rath

Birthday: 1997-03-06

Address: 14955 Ledner Trail, East Rodrickfort, NE 85127-8369

Phone: +18682428114917

Job: National Technology Representative

Hobby: Sand art, Drama, Web surfing, Cycling, Brazilian jiu-jitsu, Leather crafting, Creative writing

Introduction: My name is Pres. Carey Rath, I am a faithful, funny, vast, joyous, lively, brave, glamorous person who loves writing and wants to share my knowledge and understanding with you.