usb: gadget: f_fs: add poll for endpoint 0

This patch adds poll function for file representing ep0.

Ability of read from or write to ep0 file is related with actual state of ffs:
- When desctiptors or strings are not written yet, POLLOUT flag is set.
- If there is any event to read, POLLIN flag is set.
- If setup request was read, POLLIN and POLLOUT flag is set, to allow
  send response (by performing I/O operation consistent with setup request
  direction) or set stall (by performing I/O operation opposite  setup
  request direction).
Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_fs.c

@@ -28,6 +28,8 @@
 #include <linux/usb/composite.h>
 #include <linux/usb/functionfs.h>
 
+#include <linux/poll.h>
+
 #include "u_fs.h"
 #include "configfs.h"
 
@@ -570,6 +572,45 @@ static long ffs_ep0_ioctl(struct file *file, unsigned code, unsigned long value)
 	return ret;
 }
 
+static unsigned int ffs_ep0_poll(struct file *file, poll_table *wait)
+{
+	struct ffs_data *ffs = file->private_data;
+	unsigned int mask = POLLWRNORM;
+	int ret;
+
+	poll_wait(file, &ffs->ev.waitq, wait);
+
+	ret = ffs_mutex_lock(&ffs->mutex, file->f_flags & O_NONBLOCK);
+	if (unlikely(ret < 0))
+		return mask;
+
+	switch (ffs->state) {
+	case FFS_READ_DESCRIPTORS:
+	case FFS_READ_STRINGS:
+		mask |= POLLOUT;
+		break;
+
+	case FFS_ACTIVE:
+		switch (ffs->setup_state) {
+		case FFS_NO_SETUP:
+			if (ffs->ev.count)
+				mask |= POLLIN;
+			break;
+
+		case FFS_SETUP_PENDING:
+		case FFS_SETUP_CANCELLED:
+			mask |= (POLLIN | POLLOUT);
+			break;
+		}
+	case FFS_CLOSING:
+		break;
+	}
+
+	mutex_unlock(&ffs->mutex);
+
+	return mask;
+}
+
 static const struct file_operations ffs_ep0_operations = {
 	.llseek =	no_llseek,
 
@@ -578,6 +619,7 @@ static const struct file_operations ffs_ep0_operations = {
 	.read =		ffs_ep0_read,
 	.release =	ffs_ep0_release,
 	.unlocked_ioctl =	ffs_ep0_ioctl,
+	.poll =		ffs_ep0_poll,
 };