serdev: use zero to indicate infinite write timeout

Use zero to indicate infinite timeout for the synchronous
serdev_device_write() helper.

This allows drivers to specify an infinite timeout without knowing about
serdev implementation details, while also allowing the same timeout
argument to be used for both serdev_device_write() and
serdev_device_wait_until_sent().

Note that passing zero to the current helper makes no sense; just call
the asynchronous serdev_device_write_buf() directly instead.
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serdev/core.c

@@ -15,6 +15,7 @@
 #include <linux/of_device.h>
 #include <linux/pm_domain.h>
 #include <linux/pm_runtime.h>
+#include <linux/sched.h>
 #include <linux/serdev.h>
 #include <linux/slab.h>
 
@@ -235,10 +236,12 @@ int serdev_device_write(struct serdev_device *serdev,
 	struct serdev_controller *ctrl = serdev->ctrl;
 	int ret;
 
-	if (!ctrl || !ctrl->ops->write_buf ||
-	    (timeout && !serdev->ops->write_wakeup))
+	if (!ctrl || !ctrl->ops->write_buf || !serdev->ops->write_wakeup)
 		return -EINVAL;
 
+	if (timeout == 0)
+		timeout = MAX_SCHEDULE_TIMEOUT;
+
 	mutex_lock(&serdev->write_lock);
 	do {
 		reinit_completion(&serdev->write_comp);