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Subject[PATCH v4 07/11] serial: 8250: enable AFE on ports where FIFO is 16 bytes
Date
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>
---
drivers/tty/serial/8250/8250_port.c | 9 +++------
1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index d403603..be9cc37 100644
--- 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",
@@ -2550,12 +2550,9 @@ 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;
--
2.8.1
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