FAILED: patch "[PATCH] serial: uartps: Fix stuck ISR if RX disabled with non-empty" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-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]>.

thanks,

greg k-h

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

From 7abab1605139bc41442864c18f9573440f7ca105 Mon Sep 17 00:00:00 2001
From: Anssi Hannula <[email protected]>
Date: Fri, 15 Feb 2019 18:45:08 +0200
Subject: [PATCH] serial: uartps: Fix stuck ISR if RX disabled with non-empty
FIFO

If RX is disabled while there are still unprocessed bytes in RX FIFO, cdns_uart_handle_rx() called from interrupt handler will get stuck in the receive loop as read bytes will not get removed from the RX FIFO and CDNS_UART_SR_RXEMPTY bit will never get set.

Avoid the stuck handler by checking first if RX is disabled. port->lock protects against race with RX-disabling functions.

This HW behavior was mentioned by Nathan Rossi in 43e98facc4a3 ("tty:
xuartps: Fix RX hang, and TX corruption in termios call") which fixed a similar issue in cdns_uart_set_termios().
The behavior can also be easily verified by e.g. setting CDNS_UART_CR_RX_DIS at the beginning of cdns_uart_handle_rx() - the following loop will then get stuck.

Resetting the FIFO using RXRST would not set RXEMPTY either so simply issuing a reset after RX-disable would not work.

I observe this frequently on a ZynqMP board during heavy RX load at 1M baudrate when the reader process exits and thus RX gets disabled.

index 094f2958cb2b..ee9f18c52d29 100644
@@ -364,7 +364,13 @@ static irqreturn_t cdns_uart_isr(int irq, void *dev_id)
 		cdns_uart_handle_tx(dev_id);
 		isrstatus &= ~CDNS_UART_IXR_TXEMPTY;
 	}
-	if (isrstatus & CDNS_UART_IXR_RXMASK)
+
+	/*
+	 * Skip RX processing if RX is disabled as RXEMPTY will never be set
+	 * as read bytes will not be removed from the FIFO.
+	 */
+	if (isrstatus & CDNS_UART_IXR_RXMASK &&
+	    !(readl(port->membase + CDNS_UART_CR) & CDNS_UART_CR_RX_DIS))
 		cdns_uart_handle_rx(dev_id, isrstatus);
 
 	spin_unlock(&port->lock);