serial: 8250: enable AFE on ports where FIFO is 16 bytes

Intel Quark has 16550A compatible UART with autoflow feature enabled. It has
only 16 bytes of FIFO. Currently serial8250_do_set_termios() prevents to enable
autoflow since the minimum requirement of 32 bytes of FIFO size.

Drop a FIFO size limitation to allow autoflow control be enabled on such UARTs.

While here, comment out UART_CAP_AFE for PORT_AR7 since it wasn't working and
it will be not a good idea to use it in conjunction with trigger level of 1
byte.

Suggested-by: Peter Hurley <peter@hurleysoftware.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
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
index c45c1e3..1bd7ea0 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -178,7 +178,7 @@ static const struct serial8250_config uart_config[] = {
                .fifo_size      = 16,
                .tx_loadsz      = 16,
                .fcr            = UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_00,
-               .flags          = UART_CAP_FIFO | UART_CAP_AFE,
+               .flags          = UART_CAP_FIFO /* | UART_CAP_AFE */,
        },
        [PORT_U6_16550A] = {
                .name           = "U6_16550A",
@@ -2549,12 +2549,9 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
        /*
         * MCR-based auto flow control.  When AFE is enabled, RTS will be
         * deasserted when the receive FIFO contains more characters than
-        * the trigger, or the MCR RTS bit is cleared.  In the case where
-        * the remote UART is not using CTS auto flow control, we must
-        * have sufficient FIFO entries for the latency of the remote
-        * UART to respond.  IOW, at least 32 bytes of FIFO.
+        * the trigger, or the MCR RTS bit is cleared.
         */
-       if (up->capabilities & UART_CAP_AFE && port->fifosize >= 32) {
+       if (up->capabilities & UART_CAP_AFE) {
                up->mcr &= ~UART_MCR_AFE;
                if (termios->c_cflag & CRTSCTS)
                        up->mcr |= UART_MCR_AFE;