serial: 8250_pci: Remove custom deprecated baud setting routine

As comments mentioned this is old (and actually deprecated) interface
to set custom baud rates. This interface has limitations as it only
allows to set a single opaque value called "custom_divisor". If the HW
needs more complex settings (like fractional divisor) it must somehow
encode this. This is horrid interface that is very driver specific
and not flexible. Meanwhile Linux has established way to set free
baud rate settings via BOTHER [1]. With all this being said, remove
deprecated interface for good.

Link: https://stackoverflow.com/questions/12646324/how-can-i-set-a-custom-baud-rate-on-linux [1]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20260122102349.2395423-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: andy-shev

Documentation/misc-devices/oxsemi-tornado.rst

@@ -89,31 +89,7 @@ With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
 limitation imposed by ``serial8250_get_baud_rate`` standard baud rates
 below 300bps become unavailable in the regular way, e.g. the rate of
 200bps requires the baud base to be divided by 78125 and that is beyond
-the unsigned 16-bit range.  The historic spd_cust feature can still be
-used by encoding the values for, the prescaler, the oversampling rate
-and the clock divisor (DLM/DLL) as follows to obtain such rates if so
-required:
-
-::
-
-  31 29 28             20 19   16 15                            0
- +-----+-----------------+-------+-------------------------------+
- |0 0 0|    CPR2:CPR     |  TCR  |            DLM:DLL            |
- +-----+-----------------+-------+-------------------------------+
-
-Use a value such encoded for the ``custom_divisor`` field along with the
-ASYNC_SPD_CUST flag set in the ``flags`` field in ``struct serial_struct``
-passed with the TIOCSSERIAL ioctl(2), such as with the setserial(8)
-utility and its ``divisor`` and ``spd_cust`` parameters, and then select
-the baud rate of 38400bps.  Note that the value of 0 in TCR sets the
-oversampling rate to 16 and prescaler values below 1 in CPR2/CPR are
-clamped by the driver to 1.
-
-For example the value of 0x1f4004e2 will set CPR2/CPR, TCR and DLM/DLL
-respectively to 0x1f4, 0x0 and 0x04e2, choosing the prescaler value,
-the oversampling rate and the clock divisor of 62.500, 16 and 1250
-respectively.  These parameters will set the baud rate for the serial
-port to 62500000 / 62.500 / 1250 / 16 = 50bps.
+the unsigned 16-bit range.
 
 Maciej W. Rozycki  <macro@orcam.me.uk>
 

drivers/tty/serial/8250/8250_pci.c

@@ -1205,60 +1205,49 @@ static unsigned int pci_oxsemi_tornado_get_divisor(struct uart_port *port,
 	u8 tcr;
 	int i;
 
-	/* Old custom speed handling.  */
-	if (baud == 38400 && (port->flags & UPF_SPD_MASK) == UPF_SPD_CUST) {
-		unsigned int cust_div = port->custom_divisor;
-
-		quot = cust_div & UART_DIV_MAX;
-		tcr = (cust_div >> 16) & OXSEMI_TORNADO_TCR_MASK;
-		cpr = (cust_div >> 20) & OXSEMI_TORNADO_CPR_MASK;
-		if (cpr < OXSEMI_TORNADO_CPR_MIN)
-			cpr = OXSEMI_TORNADO_CPR_DEF;
-	} else {
-		best_squot = quot_scale;
-		for (i = 0; i < ARRAY_SIZE(p); i++) {
-			unsigned int spre;
-			unsigned int srem;
-			u8 cp;
-			u8 tc;
-
-			tc = p[i][0];
-			cp = p[i][1];
-			spre = tc * cp;
-
-			srem = sdiv % spre;
-			if (srem > spre / 2)
-				srem = spre - srem;
-			squot = DIV_ROUND_CLOSEST(srem * quot_scale, spre);
-
-			if (srem == 0) {
-				tcr = tc;
-				cpr = cp;
-				quot = sdiv / spre;
-				break;
-			} else if (squot < best_squot) {
-				best_squot = squot;
-				tcr = tc;
-				cpr = cp;
-				quot = DIV_ROUND_CLOSEST(sdiv, spre);
-			}
+	best_squot = quot_scale;
+	for (i = 0; i < ARRAY_SIZE(p); i++) {
+		unsigned int spre;
+		unsigned int srem;
+		u8 cp;
+		u8 tc;
+
+		tc = p[i][0];
+		cp = p[i][1];
+		spre = tc * cp;
+
+		srem = sdiv % spre;
+		if (srem > spre / 2)
+			srem = spre - srem;
+		squot = DIV_ROUND_CLOSEST(srem * quot_scale, spre);
+
+		if (srem == 0) {
+			tcr = tc;
+			cpr = cp;
+			quot = sdiv / spre;
+			break;
+		} else if (squot < best_squot) {
+			best_squot = squot;
+			tcr = tc;
+			cpr = cp;
+			quot = DIV_ROUND_CLOSEST(sdiv, spre);
 		}
-		while (tcr <= (OXSEMI_TORNADO_TCR_MASK + 1) >> 1 &&
-		       quot % 2 == 0) {
+	}
+	while (tcr <= (OXSEMI_TORNADO_TCR_MASK + 1) >> 1 &&
+	       quot % 2 == 0) {
+		quot >>= 1;
+		tcr <<= 1;
+	}
+	while (quot > UART_DIV_MAX) {
+		if (tcr <= (OXSEMI_TORNADO_TCR_MASK + 1) >> 1) {
 			quot >>= 1;
 			tcr <<= 1;
-		}
-		while (quot > UART_DIV_MAX) {
-			if (tcr <= (OXSEMI_TORNADO_TCR_MASK + 1) >> 1) {
-				quot >>= 1;
-				tcr <<= 1;
-			} else if (cpr <= OXSEMI_TORNADO_CPR_MASK >> 1) {
-				quot >>= 1;
-				cpr <<= 1;
-			} else {
-				quot = quot * cpr / OXSEMI_TORNADO_CPR_MASK;
-				cpr = OXSEMI_TORNADO_CPR_MASK;
-			}
+		} else if (cpr <= OXSEMI_TORNADO_CPR_MASK >> 1) {
+			quot >>= 1;
+			cpr <<= 1;
+		} else {
+			quot = quot * cpr / OXSEMI_TORNADO_CPR_MASK;
+			cpr = OXSEMI_TORNADO_CPR_MASK;
 		}
 	}