Input userspace API (UAPI)

Input event UAPI (<tt>RT_INPUT_UAPI</tt>)

**input_uapi.c** exposes each **rt_input_device** as a Linux-compatible character device for POSIX userspace (Smart / LWP + DFS). IOCTLs and **struct input_event** are defined in **drivers/input_uapi.h** (aligned with **include/uapi/linux/input.h**).

DM overview and driver binding: Input device model (DM). In-kernel reporters and handlers: Input subsystem.

Sources: **components/drivers/input/input_uapi.c**, **components/drivers/include/drivers/input_uapi.h**.

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Kconfig

Option	Default	Role
**RT_INPUT_UAPI**	y if **RT_USING_SMART**, else n	Build **input_uapi.c**, attach **dfs_file_ops** in **input_uapi_init**
**RT_INPUT_UAPI_EVENT_MAX**	128	Ring buffer slots (**struct input_event** per device)
**RT_UAPI_FAKE_BLOCK**	y	**O_NONBLOCK** empty **read** returns synthetic **SYN_REPORT** instead of **-EAGAIN**

Dependencies: **RT_USING_INPUT**, **RT_USING_KTIME**, **RT_USING_POSIX_DEVIO**.

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Architecture

rt_input_report_* → rt_input_sync
      → rt_input_event (input.c)
            → input_uapi_event (if ref_count > 0)
                  ring buffer + ktime timestamp
                  SYN_REPORT → sync_count++, wake wait_queue
      → read() / poll(POLLIN) on /dev/inputN

Hook	When
**input_uapi_init**	**rt_input_device_register** — allocates **struct input_uapi**, sets **idev->parent.fops**
**input_uapi_event**	Every **rt_input_event** while UAPI enabled
**input_uapi_finit**	**rt_input_device_unregister** — free UAPI state

Registered device names are **input0**, **input1**, … (**rt_dm_ida** in **input.c**). Open via devfs:

int fd = open("/dev/input0", O_RDONLY);

**input_uapi_event** returns without queuing when **idev->parent.ref_count == 0** — userspace must **open** the node before events appear.

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Event format

	Kernel handler path	UAPI queue
Struct	**struct rt_input_event** (drivers/input.h)	**struct input_event** (input_uapi.h)
Time	**rt_tick_t tick**	**struct timeval** from **rt_ktime_boottime_get_us**
Codes	**dt-bindings/input/event-codes.h**	Same

**struct input_absinfo** returned by **EVIOCGABS** matches **struct rt_input_absinfo** (**RT_ASSERT** on layout in **input_uapi_init**).

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Ring buffer and framing

**struct input_uapi** holds:

• Atomic **read_idx**, **write_idx**, **sync_count**
• **events[RT_INPUT_UAPI_EVENT_MAX]**
• Optional **grabbed_file** for **EVIOCGRAB**

On each kernel event copy, **write_idx** advances. When **type == EV_SYN** and **code == SYN_REPORT**, **sync_count** increments and **rt_wqueue_wakeup** runs.

**read** drains events until the user buffer is full or a **SYN_REPORT** completes one frame; each consumed frame decrements **sync_count**.

Overflow: if the next write would overwrite unread data, the event is dropped (warning log). Raise **RT_INPUT_UAPI_EVENT_MAX** for high-rate touch streams.

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<tt>read()</tt> / <tt>poll()</tt> / <tt>write()</tt>

Behavior	Detail
Min size	**count != 0** and **count < sizeof(struct input_event)** → **-EINVAL**
**count == 0**	Allowed (availability check after wait)
Blocking	Waits on **idev->parent.wait_queue** until **sync_count > 0**
**O_NONBLOCK**	Empty queue → **-EAGAIN**, or fake **SYN_REPORT** if **RT_UAPI_FAKE_BLOCK**
**poll**	**POLLIN** when **sync_count > 0** (**O_RDONLY** / **O_RDWR** only)
**write**	**-ENOSYS**

Grab: if another **dfs_file** holds **EVIOCGRAB**, **read** returns **-EAGAIN**.

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<tt>ioctl()</tt> (implemented)

IOCTL	Purpose
**EVIOCGVERSION**	**EV_VERSION** (0x010001)
**EVIOCGID**	Virtual ID: bustype **0x06**, vendor **0x5354**, product **0x4556**, version **RT_VER_NUM >> 16**
**EVIOCGNAME(len)**	Device name (**inputN**)
**EVIOCGPROP(len)**	Property bitmap (direct-touch bit when buffer large enough)
**EVIOCGBIT(ev, len)**	Type bitmap (**ev==0**) or **EV_KEY/EV_REL/EV_ABS** maps
**EVIOCGABS(abs)**	Axis **struct input_absinfo**
**EVIOCGRAB**	Exclusive grab; non-zero **args** grabs; second grabber **-EBUSY**

**EVIOCSABS**, force-feedback, and other Linux input IOCTLs → **-EINVAL**.

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Userspace example

#include <fcntl.h>
#include <unistd.h>
#include <poll.h>
#include <drivers/input_uapi.h>
 
void listen_input0(void)
{
    struct input_event ev[64];
    struct pollfd pfd = { .fd = open("/dev/input0", O_RDONLY), .events = POLLIN };
 
    for (;;)
    {
        if (poll(&pfd, 1, -1) <= 0)
            continue;
 
        ssize_t n = read(pfd.fd, ev, sizeof(ev));
        if (n <= 0)
            continue;
 
        for (int i = 0; i < n / (int)sizeof(struct input_event); ++i)
        {
            /* ev[i].type, ev[i].code, ev[i].value */
        }
    }
}

Use **rt_input_add_handler** in kernel; use UAPI when a POSIX process (e.g. **evtest**) reads the same event stream.

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Pitfalls

Issue	Mitigation
No events before open	**ref_count** gate in **input_uapi_event**
Unregister with open fd	**rt_input_device_unregister** checks **ref_count** — close fds first
Grab conflicts	Release with **EVIOCGRAB** and zero argument
Touch bursts	Increase **RT_INPUT_UAPI_EVENT_MAX** or read faster
**RT_UAPI_FAKE_BLOCK**	Disable for strict **-EAGAIN** on empty non-blocking reads

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See also

• Input device model (DM)
• Input subsystem
• components/drivers/include/drivers/input_uapi.h
• components/drivers/input/Kconfig