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Date: Thu, 22 Jan 2015 12:46:35 -0500
From: Peter Hurley <peter@...leysoftware.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC: Jiri Slaby <jslaby@...e.cz>, linux-serial@...r.kernel.org,
linux-kernel@...r.kernel.org, Joe Schultz <jschultz@...-inc.com>,
Aaron Sierra <asierra@...-inc.com>
Subject: Re: [PATCH 5/7] serial: 8250: Refactor XR17V35X divisor calculation
On 01/22/2015 12:24 PM, Peter Hurley wrote:
> Exar XR17V35X PCIe uarts support a 4-bit fractional divisor register.
> Refactor the divisor calculation from the divisor programming.
>
> Allow a fractional result from serial8250_get_divisor() and pass this
> result to serial8250_dl_write().
>
> Simplify the calculation for quot and quot_frac. This was verified
> to be identical to the results of the original calculation with a test
> jig.
>
> NB: The results were also compared with the divisor value chart
> on pg 33 of the Exar XR17V352 datasheet, rev 1.0.3, here:
> http://www.exar.com/common/content/document.ashx?id=1585
> which differs from the calculated values by 1 in the fractional result.
> This is because the calculated values are still rounded in the
> fractional result, whereas the table values are truncated. Note
> that the data error rate % values in the datasheet are for
> rounded fractional results, as the truncated fractional results
> have more error.
>
> Cc: Joe Schultz <jschultz@...-inc.com>
> Cc: Aaron Sierra <asierra@...-inc.com>
> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
FWIW, here's the test jig:
--- /dev/null 2015-01-22 11:30:11.407707482 -0500
+++ divisor.c 2015-01-22 12:42:34.722403359 -0500
@@ -0,0 +1,92 @@
+/*
+ * divisor.c - test jig for evaluating computation equivalence
+ * of divisors for XR17V35x UART
+ */
+
+#include <stdio.h>
+
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof(x[0]))
+
+/* from include/linux/kernel.h */
+
+#define DIV_ROUND_CLOSEST(x, divisor)( \
+{ \
+ typeof(x) __x = x; \
+ typeof(divisor) __d = divisor; \
+ (((typeof(x))-1) > 0 || \
+ ((typeof(divisor))-1) > 0 || (__x) > 0) ? \
+ (((__x) + ((__d) / 2)) / (__d)) : \
+ (((__x) - ((__d) / 2)) / (__d)); \
+} \
+)
+
+/* 125MHz clock on XR17V35x */
+unsigned int clk = 125000000;
+
+
+void calc_quot(unsigned int baud, unsigned int *quot, unsigned int *frac) {
+
+ unsigned int baud_x32 = (2 * clk) / baud;
+
+ *quot = baud_x32 / 32;
+ *frac = DIV_ROUND_CLOSEST(baud_x32 % 32, 2);
+}
+
+void calc_quot2(unsigned int baud, unsigned int *quot, unsigned int *frac) {
+
+ /* compute quotient as a 16.4 fixed point value */
+ unsigned int quot_16 = DIV_ROUND_CLOSEST(clk, baud);
+
+ *quot = quot_16 >> 4;
+ *frac = quot_16 & 0x0f;
+}
+
+
+int main() {
+ int i;
+ unsigned int bauds[] = { 2400,
+ 4800,
+ 9600,
+ 10000,
+ 19200,
+ 25000,
+ 28800,
+ 38400,
+ 50000,
+ 57600,
+ 75000,
+ 100000,
+ 115200,
+ 153600,
+ 200000,
+ 225000,
+ 230400,
+ 250000,
+ 300000,
+ 400000,
+ 460800,
+ 500000,
+ 576000,
+ 750000,
+ 921600,
+ 1000000,
+ 1152000,
+ };
+
+ for (i = 0; i < ARRAY_SIZE(bauds); i++) {
+ unsigned int quot, frac;
+ unsigned int quot2, frac2;
+
+ calc_quot(bauds[i], ", &frac);
+ calc_quot2(bauds[i], "2, &frac2);
+
+ if (quot != quot2 || frac != frac2) {
+ printf("diff: %04x.%01x != %04x.%01x\n", quot, frac,
+ quot2, frac2);
+ } else
+ printf("%10d %02X %02X %01X\n", bauds[i],
+ quot2 >> 8, quot2 & 0xff, frac2);
+ }
+
+ return 0;
+}
--
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