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Embedded Linux Learning

poll_wait

`poll_wait()`¶

Purpose¶

poll_wait() connects a driver's wait queue to the kernel poll infrastructure. It allows poll, select, and epoll to wait for driver readiness.

Prototype Pattern¶

static __poll_t mydev_poll(struct file *file, poll_table *wait)
{
    poll_wait(file, &dev->wq, wait);

    if (data_available(dev))
        return EPOLLIN | EPOLLRDNORM;

    return 0;
}

Important Concept¶

poll_wait() does not check readiness. It only registers the wait queue. The driver must still return a readiness mask based on current state.

Relationship with Wait Queues¶

A typical poll flow is:

Userspace poll()
        ↓
Driver .poll()
        ↓
poll_wait()
        ↓
Register wait queue
        ↓
Return event mask
        ↓
No event?
        ↓
Kernel sleeps
        ↓
wake_up()
        ↓
Driver .poll() again
        ↓
Return POLLIN

Notice that poll_wait() never blocks.

Sleeping is performed later by the poll framework if no file descriptor reports readiness.

Readiness Masks¶

Mask	Meaning
`EPOLLIN`	Data is available to read
`EPOLLRDNORM`	Normal readable data is available
`EPOLLOUT`	Device can accept write data
`EPOLLERR`	Error condition
`EPOLLHUP`	Hangup condition