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Subjectmake 16C950 UARTs work
This patch has been submitted a number of times, and doesn't seem
to get any upstream traction, which is a shame, as it seems to work
for users, and I keep inadvertantly dropping it from the Fedora
kernel everytime I rebase it.

http://marc.theaimsgroup.com/?l=linux-kernel&m=112687270832687&w=2
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=126403

From: Mathias Adam <a2@adamis.de>
Signed-off-by: Dave Jones <davej@redhat.com>


--- linux-2.6.13-org/drivers/serial/8250.c 2005-08-29 01:41:01.000000000 +0200
+++ linux-2.6.13/drivers/serial/8250.c 2005-09-16 12:18:14.000000000 +0200
@@ -7,6 +7,9 @@
*
* Copyright (C) 2001 Russell King.
*
+ * 2005/09/16: Enabled higher baud rates for 16C95x.
+ * (Mathias Adam <a2@adamis.de>)
+ *
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
@@ -1652,6 +1655,14 @@
else if ((port->flags & UPF_MAGIC_MULTIPLIER) &&
baud == (port->uartclk/8))
quot = 0x8002;
+ /*
+ * For 16C950s UART_TCR is used in combination with divisor==1
+ * to achieve baud rates up to baud_base*4.
+ */
+ else if ((port->type == PORT_16C950) &&
+ baud > (port->uartclk/16))
+ quot = 1;
+
else
quot = uart_get_divisor(port, baud);

@@ -1665,7 +1676,7 @@
struct uart_8250_port *up = (struct uart_8250_port *)port;
unsigned char cval, fcr = 0;
unsigned long flags;
- unsigned int baud, quot;
+ unsigned int baud, quot, max_baud;

switch (termios->c_cflag & CSIZE) {
case CS5:
@@ -1697,7 +1708,8 @@
/*
* Ask the core to calculate the divisor for us.
*/
- baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk/16);
+ max_baud = (up->port.type == PORT_16C950 ? port->uartclk/4 : port->uartclk/16);
+ baud = uart_get_baud_rate(port, termios, old, 0, max_baud);
quot = serial8250_get_divisor(port, baud);

/*
@@ -1733,6 +1745,19 @@
*/
spin_lock_irqsave(&up->port.lock, flags);

+ /*
+ * 16C950 supports additional prescaler ratios between 1:16 and 1:4
+ * thus increasing max baud rate to uartclk/4.
+ */
+ if (up->port.type == PORT_16C950) {
+ if (baud == port->uartclk/4)
+ serial_icr_write(up, UART_TCR, 0x4);
+ else if (baud == port->uartclk/8)
+ serial_icr_write(up, UART_TCR, 0x8);
+ else
+ serial_icr_write(up, UART_TCR, 0);
+ }
+
/*
* Update the per-port timeout.
*/

--
http://www.codemonkey.org.uk
-
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