powerpc: Honor O_NONBLOCK flag when reading RTAS log

rtas_log_read() doesn't check file flags for O_NONBLOCK and blocks
non-blocking readers of /proc/ppc64/rtas/error_log when there is
no data available. This fixes it.

Also rtas_log_read() returns now with ENODATA to prevent suspending of
process in wait_event_interruptible() when logging facility was
switched off and log is already empty.
Signed-off-by: NVitaly Mayatskikh <v.mayatskih@gmail.com>
Acked-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/rtasd.c

@@ -295,19 +295,29 @@ static ssize_t rtas_log_read(struct file * file, char __user * buf,
 	if (!tmp)
 		return -ENOMEM;
 
-
 	spin_lock_irqsave(&rtasd_log_lock, s);
 	/* if it's 0, then we know we got the last one (the one in NVRAM) */
-	if (rtas_log_size == 0 && logging_enabled)
-		nvram_clear_error_log();
+	while (rtas_log_size == 0) {
+		if (file->f_flags & O_NONBLOCK) {
 			spin_unlock_irqrestore(&rtasd_log_lock, s);
+			error = -EAGAIN;
+			goto out;
+		}
 
+		if (!logging_enabled) {
+			spin_unlock_irqrestore(&rtasd_log_lock, s);
+			error = -ENODATA;
+			goto out;
+		}
+		nvram_clear_error_log();
 
+		spin_unlock_irqrestore(&rtasd_log_lock, s);
 		error = wait_event_interruptible(rtas_log_wait, rtas_log_size);
 		if (error)
 			goto out;
-
 		spin_lock_irqsave(&rtasd_log_lock, s);
+	}
+
 	offset = rtas_error_log_buffer_max * (rtas_log_start & LOG_NUMBER_MASK);
 	memcpy(tmp, &rtas_log_buf[offset], count);