Revert "serial: 8250: Fix clearing FIFOs in RS485 mode again"

paulburton · gregkh · commit 3c9dc275dba1 · 2018-12-17T16:18:29.000+01:00
Commit f6aa5be ("serial: 8250: Fix clearing FIFOs in RS485 mode again") makes a change to FIFO clearing code which its commit message suggests was intended to be specific to use with RS485 mode, however: 1) The change made does not just affect __do_stop_tx_rs485(), it also affects other uses of serial8250_clear_fifos() including paths for starting up, shutting down or auto-configuring a port regardless of whether it's an RS485 port or not. 2) It makes the assumption that resetting the FIFOs is a no-op when FIFOs are disabled, and as such it checks for this case & explicitly avoids setting the FIFO reset bits when the FIFO enable bit is clear. A reading of the PC16550D manual would suggest that this is OK since the FIFO should automatically be reset if it is later enabled, but we support many 16550-compatible devices and have never required this auto-reset behaviour for at least the whole git era. Starting to rely on it now seems risky, offers no benefit, and indeed breaks at least the Ingenic JZ4780's UARTs which reads garbage when the RX FIFO is enabled if we don't explicitly reset it. 3) By only resetting the FIFOs if they're enabled, the behaviour of serial8250_do_startup() during boot now depends on what the value of FCR is before the 8250 driver is probed. This in itself seems questionable and leaves us with FCR=0 & no FIFO reset if the UART was used by 8250_early, otherwise it depends upon what the bootloader left behind. 4) Although the naming of serial8250_clear_fifos() may be unclear, it is clear that callers of it expect that it will disable FIFOs. Both serial8250_do_startup() & serial8250_do_shutdown() contain comments to that effect, and other callers explicitly re-enable the FIFOs after calling serial8250_clear_fifos(). The premise of that patch that disabling the FIFOs is incorrect therefore seems wrong. For these reasons, this reverts commit f6aa5be ("serial: 8250: Fix clearing FIFOs in RS485 mode again"). Signed-off-by: Paul Burton <paul.burton@mips.com> Fixes: f6aa5be ("serial: 8250: Fix clearing FIFOs in RS485 mode again"). Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Daniel Jedrychowski <avistel@gmail.com> Cc: Marek Vasut <marex@denx.de> Cc: linux-mips@vger.kernel.org Cc: linux-serial@vger.kernel.org Cc: stable <stable@vger.kernel.org> # 4.10+ Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
@@ -552,30 +552,11 @@ static unsigned int serial_icr_read(struct uart_8250_port *up, int offset)
  */
 static void serial8250_clear_fifos(struct uart_8250_port *p)
 {
-	unsigned char fcr;
-	unsigned char clr_mask = UART_FCR_CLEAR_RCVR | UART_FCR_CLEAR_XMIT;
-
 	if (p->capabilities & UART_CAP_FIFO) {
-		/*
-		 * Make sure to avoid changing FCR[7:3] and ENABLE_FIFO bits.
-		 * In case ENABLE_FIFO is not set, there is nothing to flush
-		 * so just return. Furthermore, on certain implementations of
-		 * the 8250 core, the FCR[7:3] bits may only be changed under
-		 * specific conditions and changing them if those conditions
-		 * are not met can have nasty side effects. One such core is
-		 * the 8250-omap present in TI AM335x.
-		 */
-		fcr = serial_in(p, UART_FCR);
-
-		/* FIFO is not enabled, there's nothing to clear. */
-		if (!(fcr & UART_FCR_ENABLE_FIFO))
-			return;
-
-		fcr |= clr_mask;
-		serial_out(p, UART_FCR, fcr);
-
-		fcr &= ~clr_mask;
-		serial_out(p, UART_FCR, fcr);
+		serial_out(p, UART_FCR, UART_FCR_ENABLE_FIFO);
+		serial_out(p, UART_FCR, UART_FCR_ENABLE_FIFO |
+			       UART_FCR_CLEAR_RCVR | UART_FCR_CLEAR_XMIT);
+		serial_out(p, UART_FCR, 0);
 	}
 }
 
@@ -1467,7 +1448,7 @@ static void __do_stop_tx_rs485(struct uart_8250_port *p)
 	 * Enable previously disabled RX interrupts.
 	 */
 	if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX)) {
-		serial8250_clear_fifos(p);
+		serial8250_clear_and_reinit_fifos(p);
 
 		p->ier |= UART_IER_RLSI | UART_IER_RDI;
 		serial_port_out(&p->port, UART_IER, p->ier);
