Re: [PATCH v2] Documentation: Input: Add uinput documentation

[Peter Hutterer]

as usual, reading through these things multiple times means one spots a
couple of different things. sorry about that.

On Fri, Mar 24, 2017 at 12:34:59AM -0300, Marcos Paulo de Souza wrote:
> Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@xxxxxxxxx>
> ---
>  Documentation/input/uinput.rst | 196 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 196 insertions(+)
>  create mode 100644 Documentation/input/uinput.rst
> 
> diff --git a/Documentation/input/uinput.rst b/Documentation/input/uinput.rst
> new file mode 100644
> index 0000000..eb79b77
> --- /dev/null
> +++ b/Documentation/input/uinput.rst
> @@ -0,0 +1,196 @@
> +=============
> +uinput module
> +=============
> +
> +Introduction
> +============
> +
> +uinput is a kernel module that makes possible to create and handle input devices

typo: makes *it* possible.

replace "to create and handle" with "to emulate", the rest is in the next
sentence anyway

> +from userspace. By writing to the module's /dev/uinput (or /dev/input/uinput), a
> +process can create a virtual device with specific capabilities.
> +Once created, the process can send events through that virtual device.
> +
> +Interface
> +=========
> +
> +::
> +
> +  linux/uinput.h
> +
> +The uinput header defines ioctls to create, setup and destroy virtual devices.
> +
> +libevdev
> +========
> +
> +libevdev is a wrapper library for evdev devices, making uinput setup easier
> +by skipping a lot of ioctl calls. When dealing with uinput, libevdev is the best
> +alternative over accessing uinput directly, and it is less error prone.

"libevdev is a wrapper library for evdev devices that provides interfaces to
create uinput devices and send events. libevdev is less error-prone than
accessing uinput directly and should be considered for new software".

> +
> +For examples and more information about libevdev:
> +https://cgit.freedesktop.org/libevdev
> +

Please use https://www.freedesktop.org/software/libevdev/doc/latest/
(which needs a link to the git repo, I'll fix that in a minute)

> +Examples
> +========
> +
> +1.0 Keyboard events
> +-------------------
> +
> +This first example shows how to create a new virtual device and how to send a
> +key event. All default imports and error handlers were removed for the sake of
> +simplicity.
> +
> +.. code-block:: c
> +
> +   #include <linux/uinput.h>
> +
> +   int fd;
> +
> +   void emit(int type, int code, int val)
> +   {
> +        struct input_event ie;
> +
> +        ie.type = type;
> +        ie.code = code;
> +        ie.value = val;
> +        /* below timestamp values are ignored */
> +        ie.time.tv_sec = 0;
> +        ie.time.tv_usec = 0;
> +
> +        write(fd, &ie, sizeof(ie));
> +   }
> +
> +   int main() {
> +        struct uinput_setup usetup;
> +
> +        fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK);
> +
> +        /* the ioctls below enables the to be created device to key
> +         * events, in this case the space key
> +         **/

the comment terminator doesn't look right

> +        ioctl(fd, UI_SET_EVBIT, EV_KEY);
> +        ioctl(fd, UI_SET_KEYBIT, KEY_SPACE);
> +
> +        memset(&usetup, 0, sizeof(usetup));
> +        usetup.id.bustype = BUS_USB;
> +        usetup.id.vendor = 0x1234; /* sample vendor */

add a sample product id too please

> +        strcpy(usetup.name, "Example device");
> +
> +        ioctl(fd, UI_DEV_SETUP, &usetup);
> +        ioctl(fd, UI_DEV_CREATE);
> +
> +        /* UI_DEV_CREATE causes the kernel to create the device nodes for this

"On UI_DEV_CREATE the kernel creates the device nodes..."

> +         * device. Insert a pause so that userspace has time to detect,
> +         * initialize the new device, and can start to listen to events from
> +         * this device
> +         **/

the comment terminator doesn't look right

note: the actual pause is missing now :)

> +
> +        /* key press, report the event, send key release, and report again */
> +        emit(EV_KEY, KEY_SPACE, 1);
> +        emit(EV_SYN, SYN_REPORT, 0);
> +        emit(EV_KEY, KEY_SPACE, 0);
> +        emit(EV_SYN, SYN_REPORT, 0);

come to think of it, you probably need a pause here too, iirc a caller may
get ENODEV before reading the key events otherwise.

> +
> +        ioctl(fd, UI_DEV_DESTROY);
> +        close(fd);
> +
> +        return 0;
> +   }
> +
> +2.0 Mouse movements
> +-------------------
> +
> +This example shows how to create a virtual device that behaves like a physical
> +mouse.
> +
> +.. code-block:: c
> +
> +    #include <linux/uinput.h>
> +
> +    /* emit function is identical to of the first example */
> +
> +    struct uinput_setup usetup;
> +    int i = 50;
> +
> +    fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK);
> +
> +    /* enable mouse button left and relative events */
> +    ioctl(fd, UI_SET_EVBIT, EV_KEY);
> +    ioctl(fd, UI_SET_KEYBIT, BTN_LEFT);
> +
> +    ioctl(fd, UI_SET_EVBIT, EV_REL);
> +    ioctl(fd, UI_SET_RELBIT, REL_X);
> +    ioctl(fd, UI_SET_RELBIT, REL_Y);
> +
> +    memset(&usetup, 0, sizeof(usetup));
> +    usetup.id.bustype = BUS_USB;
> +    usetup.id.vendor = 0x1234; /* sample vendor */
> +    strcpy(usetup.name, "Example device");
> +
> +    ioctl(fd, UI_DEV_SETUP, &usetup);
> +    ioctl(fd, UI_DEV_CREATE);
> +
> +    /* UI_DEV_CREATE causes the kernel to create the device nodes for this
> +     * device. Insert a pause so that userspace has time to detect,
> +     * initialize the new device, and can start to listen to events from
> +     * this device
> +     **/
> +
> +    /* moves the mouse diagonally, 5 units per axis */
> +    while (i--) {
> +        emit(EV_REL, REL_X, 5);
> +        emit(EV_REL, REL_Y, 5);
> +        emit(EV_SYN, SYN_REPORT, 0);
> +        usleep(15000);
> +    }
> +
> +    ioctl(fd, UI_DEV_DESTROY);
> +    close(fd);
> +
> +    return 0;
> +
> +3.0 uinput old interface
> +------------------------
> +
> +Before kernel 4.5, uinput didn't have an ioctl to setup a virtual device. When
> +running a version prior to 4.5, the user needs to fill a different struct and
> +call write on the uinput file descriptor.

I think this should really include the version check.

Cheers,
   Peter

> +
> +.. code-block:: c
> +
> +    #include <linux/uinput.h>
> +
> +    /* emit function is identical to of the first example */
> +
> +    struct uinput_user_dev uud;
> +
> +    fd = open("/dev/uinput", O_WRONLY | O_NONBLOCK);
> +
> +    /* the ioctls below enables the to be created device to key
> +     * events, in this case the space key
> +     **/
> +    ioctl(fd, UI_SET_EVBIT, EV_KEY);
> +    ioctl(fd, UI_SET_KEYBIT, KEY_SPACE);
> +
> +    memset(&uud, 0, sizeof(uud));
> +    snprintf(uud.name, UINPUT_MAX_NAME_SIZE, "uinput old interface");
> +    write(fd, &uud, sizeof(uud));
> +
> +    ioctl(fd, UI_DEV_CREATE);
> +
> +    /* UI_DEV_CREATE causes the kernel to create the device nodes for this
> +     * device. Insert a pause so that userspace has time to detect,
> +     * initialize the new device, and can start to listen to events from
> +     * this device
> +     **/
> +
> +    /* key press, report the event, send key release, and report again */
> +    emit(EV_KEY, KEY_SPACE, 1);
> +    emit(EV_SYN, SYN_REPORT, 0);
> +    emit(EV_KEY, KEY_SPACE, 0);
> +    emit(EV_SYN, SYN_REPORT, 0);
> +
> +    ioctl(fd, UI_DEV_DESTROY);
> +    close(fd);
> +
> +    return 0;
> +
> -- 
> 2.9.3
>