FAILED: patch "[PATCH] serial: 8250: Fix serial8250_tx_empty() race with DMA Tx" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-4.19.y
git checkout FETCH_HEAD
git cherry-pick -x 146a37e05d620cef4ad430e5d1c9c077fe6fa76f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2023050629-nastily-commodity-a7ad@gregkh' --subject-prefix 'PATCH 4.19.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 146a37e05d620cef4ad430e5d1c9c077fe6fa76f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ilpo=20J=C3=A4rvinen?= <[email protected]>
Date: Fri, 17 Mar 2023 13:33:18 +0200
Subject: [PATCH] serial: 8250: Fix serial8250_tx_empty() race with DMA Tx
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

There's a potential race before THRE/TEMT deasserts when DMA Tx is starting up (or the next batch of continuous Tx is being submitted). This can lead to misdetecting Tx empty condition.

It is entirely normal for THRE/TEMT to be set for some time after the DMA Tx had been setup in serial8250_tx_dma(). As Tx side is definitely not empty at that point, it seems incorrect for serial8250_tx_empty() claim Tx is empty.

Fix the race by also checking in serial8250_tx_empty() whether there's DMA Tx active.

Note: This fix only addresses in-kernel race mainly to make using TCSADRAIN/FLUSH robust. Userspace can still cause other races but they seem userspace concurrency control problems.

index 287153d32536..1e8fe44a7099 100644
@@ -365,6 +365,13 @@ static inline void serial8250_do_prepare_rx_dma(struct uart_8250_port *p)
 	if (dma->prepare_rx_dma)
 		dma->prepare_rx_dma(p);
 }
+
+static inline bool serial8250_tx_dma_running(struct uart_8250_port *p)
+{
+	struct uart_8250_dma *dma = p->dma;
+
+	return dma && dma->tx_running;
+}
 #else
 static inline int serial8250_tx_dma(struct uart_8250_port *p)
 {

@@ -380,6 +387,11 @@ static inline int serial8250_request_dma(struct uart_8250_port *p)
 	return -1;
 }
 static inline void serial8250_release_dma(struct uart_8250_port *p) { }
+
+static inline bool serial8250_tx_dma_running(struct uart_8250_port *p)
+{
+	return false;
+}
 #endif
 
 static inline int ns16550a_goto_highspeed(struct uart_8250_port *up)

index fa43df05342b..107bcdfb119c 100644
@@ -2005,18 +2005,19 @@ static int serial8250_tx_threshold_handle_irq(struct uart_port *port)
 static unsigned int serial8250_tx_empty(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
+	unsigned int result = 0;
 	unsigned long flags;
-	u16 lsr;
 
 	serial8250_rpm_get(up);
 
 	spin_lock_irqsave(&port->lock, flags);
-	lsr = serial_lsr_in(up);
+	if (!serial8250_tx_dma_running(up) && uart_lsr_tx_empty(serial_lsr_in(up)))
+		result = TIOCSER_TEMT;
 	spin_unlock_irqrestore(&port->lock, flags);
 
 	serial8250_rpm_put(up);
 
-	return uart_lsr_tx_empty(lsr) ? TIOCSER_TEMT : 0;
+	return result;
 }
 
 unsigned int serial8250_do_get_mctrl(struct uart_port *port)