serial: 8250_omap: Fix Errata i2310 with RX FIFO level check

BugLink: https://bugs.launchpad.net/bugs/2076435

commit c128a1b upstream.

Errata i2310[0] says, Erroneous timeout can be triggered,
if this Erroneous interrupt is not cleared then it may leads
to storm of interrupts.

Commit 9d141c1 ("serial: 8250_omap: Implementation of Errata i2310")
which added the workaround but missed ensuring RX FIFO is really empty
before applying the errata workaround as recommended in the errata text.
Fix this by adding back check for UART_OMAP_RX_LVL to be 0 for
workaround to take effect.

[0] https://www.ti.com/lit/pdf/sprz536 page 23

Fixes: 9d141c1 ("serial: 8250_omap: Implementation of Errata i2310")
Cc: [email protected]
Reported-by: Vignesh Raghavendra <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Udit Kumar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Portia Stephens <[email protected]>
Signed-off-by: Roxana Nicolescu <[email protected]>

Author: uditkumarti

drivers/tty/serial/8250/8250_omap.c

@@ -673,7 +673,8 @@ static irqreturn_t omap8250_irq(int irq, void *dev_id)
 	 * https://www.ti.com/lit/pdf/sprz536
 	 */
 	if (priv->habit & UART_RX_TIMEOUT_QUIRK &&
-		(iir & UART_IIR_RX_TIMEOUT) == UART_IIR_RX_TIMEOUT) {
+	    (iir & UART_IIR_RX_TIMEOUT) == UART_IIR_RX_TIMEOUT &&
+	    serial_port_in(port, UART_OMAP_RX_LVL) == 0) {
 		unsigned char efr2, timeout_h, timeout_l;
 
 		efr2 = serial_in(up, UART_OMAP_EFR2);