Patch "imx-serial: RX DMA startup latency" has been added to the 4.12-stable tree

This is a note to let you know that I've just added the patch titled

imx-serial: RX DMA startup latency

to the 4.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
imx-serial-rx-dma-startup-latency.patch
and it can be found in the queue-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Peter Senna Tschudin <[email protected]>
Date: Sun, 14 May 2017 14:35:15 +0200
Subject: imx-serial: RX DMA startup latency

From: Peter Senna Tschudin <[email protected]>

commit 4dec2f119e86f9c91e60cdd8f0cc057452e331a9 upstream.

18a4208 introduced a change to reduce the RX DMA latency on the first reception when the serial port was opened for reading. However it was claiming a hardirq unsafe lock after a hardirq safe lock which is not allowed and causes lockdep to complain verbosely.

This patch changes the code to always start RX DMA earlier, instead of relying on the flags used to open the serial port removing the code that was looking for the serial file flags.

---
 drivers/tty/serial/imx.c |   26 +++++---------------------
 1 file changed, 5 insertions(+), 21 deletions(-)

@@ -1340,29 +1340,13 @@ static int imx_startup(struct uart_port
 	imx_enable_ms(&sport->port);
 
 	/*
-	 * If the serial port is opened for reading start RX DMA immediately
-	 * instead of waiting for RX FIFO interrupts. In our iMX53 the average
-	 * delay for the first reception dropped from approximately 35000
-	 * microseconds to 1000 microseconds.
+	 * Start RX DMA immediately instead of waiting for RX FIFO interrupts.
+	 * In our iMX53 the average delay for the first reception dropped from
+	 * approximately 35000 microseconds to 1000 microseconds.
 	 */
 	if (sport->dma_is_enabled) {
-		struct tty_struct *tty = sport->port.state->port.tty;
-		struct tty_file_private *file_priv;
-		int readcnt = 0;
-
-		spin_lock(&tty->files_lock);
-
-		if (!list_empty(&tty->tty_files))
-			list_for_each_entry(file_priv, &tty->tty_files, list)
-				if (!(file_priv->file->f_flags & O_WRONLY))
-					readcnt++;
-
-		spin_unlock(&tty->files_lock);
-
-		if (readcnt > 0) {
-			imx_disable_rx_int(sport);
-			start_rx_dma(sport);
-		}
+		imx_disable_rx_int(sport);
+		start_rx_dma(sport);
 	}
 
 	spin_unlock_irqrestore(&sport->port.lock, flags);

Patches currently in stable-queue which might be from [email protected] are

queue-4.12/imx-serial-rx-dma-startup-latency.patch