serial: 8250_pci: rewrite pericom_do_set_divisor()

BugLink: https://bugs.launchpad.net/bugs/1953731
commit bb1201d4b38ec67bd9a871cf86b0cc10f28b15b5 upstream.

Have pericom_do_set_divisor() use the uartclk instead of a hard coded
value to work with different speed crystals. Tested with 14.7456 and 24
MHz crystals.

Have pericom_do_set_divisor() always calculate the divisor rather than
call serial8250_do_set_divisor() for rates below baud_base.

Do not write registers or call serial8250_do_set_divisor() if valid
divisors could not be found.

Fixes: 6bf4e42f1d19 ("serial: 8250: Add support for higher baud rates to Pericom chips")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Jay Dolan <jay.dolan@accesio.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211122120604.3909-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

diff --git a/drivers/tty/serial/8250/8250_pci.c b/drivers/tty/serial/8250/8250_pci.c
index e6656036c50325f9fd5863e546708e389154ea28..f7d89440076a1c6ac48f09d0d8328fd1a92fe8b1 100644 (file)
--- a/drivers/tty/serial/8250/8250_pci.c
+++ b/drivers/tty/serial/8250/8250_pci.c
@@ -1349,29 +1349,33 @@ pericom_do_set_divisor(struct uart_port *port, unsigned int baud,
 {
        int scr;
        int lcr;
-       int actual_baud;
-       int tolerance;
 
-       for (scr = 5 ; scr <= 15 ; scr++) {
-               actual_baud = 921600 * 16 / scr;
-               tolerance = actual_baud / 50;
+       for (scr = 16; scr > 4; scr--) {
+               unsigned int maxrate = port->uartclk / scr;
+               unsigned int divisor = max(maxrate / baud, 1U);
+               int delta = maxrate / divisor - baud;
 
-               if ((baud < actual_baud + tolerance) &&
-                       (baud > actual_baud - tolerance)) {
+               if (baud > maxrate + baud / 50)
+                       continue;
 
+               if (delta > baud / 50)
+                       divisor++;
+
+               if (divisor > 0xffff)
+                       continue;
+
+               /* Update delta due to possible divisor change */
+               delta = maxrate / divisor - baud;
+               if (abs(delta) < baud / 50) {
                        lcr = serial_port_in(port, UART_LCR);
                        serial_port_out(port, UART_LCR, lcr | 0x80);
-
-                       serial_port_out(port, UART_DLL, 1);
-                       serial_port_out(port, UART_DLM, 0);
+                       serial_port_out(port, UART_DLL, divisor & 0xff);
+                       serial_port_out(port, UART_DLM, divisor >> 8 & 0xff);
                        serial_port_out(port, 2, 16 - scr);
                        serial_port_out(port, UART_LCR, lcr);
                        return;
-               } else if (baud > actual_baud) {
-                       break;
                }
        }
-       serial8250_do_set_divisor(port, baud, quot, quot_frac);
 }
 static int pci_pericom_setup(struct serial_private *priv,
                  const struct pciserial_board *board,