auxdisplay: Remove in_interrupt() usage.

charlcd_write() is invoked as a VFS->write() callback and as such it is
always invoked from preemptible context and may sleep.

charlcd_puts() is invoked from register/unregister callback which is
preemptible. The reboot notifier callback is also invoked from
preemptible context.

Therefore there is no need to use in_interrupt() to figure out if it
is safe to sleep because it always is. in_interrupt() and related
context checks are being removed from non-core code.
Using schedule() to schedule (and be friendly to others) is
discouraged and cond_resched() should be used instead.

Remove in_interrupt() and use cond_resched() to schedule every 32
iterations if needed.

Link: https://lkml.kernel.org/r/20200914204209.256266093@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
[mo: fixed a couple typos in comment and commit message]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Sebastian Andrzej Siewior

drivers/auxdisplay/charlcd.c

@@ -470,12 +470,14 @@ static ssize_t charlcd_write(struct file *file, const char __user *buf,
 	char c;
 
 	for (; count-- > 0; (*ppos)++, tmp++) {
-		if (!in_interrupt() && (((count + 1) & 0x1f) == 0))
+		if (((count + 1) & 0x1f) == 0) {
 			/*
-			 * let's be a little nice with other processes
-			 * that need some CPU
+			 * charlcd_write() is invoked as a VFS->write() callback
+			 * and as such it is always invoked from preemptible
+			 * context and may sleep.
 			 */
-			schedule();
+			cond_resched();
+		}
 
 		if (get_user(c, tmp))
 			return -EFAULT;
@@ -537,12 +539,8 @@ static void charlcd_puts(struct charlcd *lcd, const char *s)
 	int count = strlen(s);
 
 	for (; count-- > 0; tmp++) {
-		if (!in_interrupt() && (((count + 1) & 0x1f) == 0))
-			/*
-			 * let's be a little nice with other processes
-			 * that need some CPU
-			 */
-			schedule();
+		if (((count + 1) & 0x1f) == 0)
+			cond_resched();
 
 		charlcd_write_char(lcd, *tmp);
 	}