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Subject[PATCH] mpc5200b/uart: select more tolerant uart prescaler on low baudrates
Date
In addition to the /32 prescaler, the MPC5200B supports a second baudrate 
prescaler /4 to reach higher baudrates.
The current calculation (introduced with commit 0d1f22e4) in the kernel
preferes this low prescaler as often as possible, but with some imprecise
counterparts the communication on low baudrates fails.
According a support-mail from freescale the low prescaler (/4) allows just
1% tolerance in bittiming in contrast to 4% of the high prescaler (/32).
The prescaler not only affects the baudrate-calculation, but also the sampling
of the bits on the wire.

With this patch, we use the slightly less precise, but higher tolerant
prescaler calculation on low baudrates up to (and including) 115200 baud and
the more precise calculation above.

Tested on a custom MPC5200B board with "fsl,mpc5200b-psc-uart".

Signed-off-by: Frank Benkert <frank.benkert@avat.de>

---
Calculation Examples with prescaler (PS) 4 and 32 and divisor (DIV) on various
baudrates. Real stands for the real baudrate generated and Diff for the
differences between:
50 Baud PS 32 DIV 0xa122 Real 50 Diff 0.00%
75 Baud PS 32 DIV 0x6b6c Real 75 Diff 0.00%
110 Baud PS 32 DIV 0x493e Real 110 Diff 0.00%
134 Baud PS 32 DIV 0x3c20 Real 133 Diff 0.75%
150 Baud PS 32 DIV 0x35b6 Real 150 Diff 0.00%
200 Baud PS 32 DIV 0x2849 Real 199 Diff 0.50%
300 Baud PS 4 DIV 0xd6d8 Real 300 Diff 0.00%
PS 32 DIV 0x1adb Real 300 Diff 0.00%
600 Baud PS 4 DIV 0x6b6c Real 600 Diff 0.00%
PS 32 DIV 0x0d6e Real 599 Diff 0.17%
1200 Baud PS 4 DIV 0x35b6 Real 1200 Diff 0.00%
PS 32 DIV 0x06b7 Real 1199 Diff 0.08%
1800 Baud PS 4 DIV 0x23cf Real 1799 Diff 0.06%
PS 32 DIV 0x047a Real 1799 Diff 0.06%
2400 Baud PS 4 DIV 0x1adb Real 2400 Diff 0.00%
PS 32 DIV 0x035b Real 2401 Diff - 0.04%
4800 Baud PS 4 DIV 0x0d6e Real 4799 Diff 0.02%
PS 32 DIV 0x01ae Real 4796 Diff 0.08%
9600 Baud PS 4 DIV 0x06b7 Real 9598 Diff 0.02%
PS 32 DIV 0x00d7 Real 9593 Diff 0.07%
19200 Baud PS 4 DIV 0x035b Real 19208 Diff - 0.04%
PS 32 DIV 0x006b Real 19275 Diff - 0.39%
38400 Baud PS 4 DIV 0x01ae Real 38372 Diff 0.07%
PS 32 DIV 0x0036 Real 38194 Diff 0.54%
57600 Baud PS 4 DIV 0x011e Real 57692 Diff - 0.16%
PS 32 DIV 0x0024 Real 57291 Diff 0.54%
76800 Baud PS 4 DIV 0x00d7 Real 76744 Diff 0.07%
PS 32 DIV 0x001b Real 76388 Diff 0.54%
115200 Baud PS 4 DIV 0x008f Real 115384 Diff - 0.16%
PS 32 DIV 0x0012 Real 114583 Diff 0.54%
153600 Baud PS 4 DIV 0x006b Real 154205 Diff - 0.39%
PS 32 DIV 0x000d Real 158653 Diff - 3.29%
230400 Baud PS 4 DIV 0x0048 Real 229166 Diff 0.54%
PS 32 DIV 0x0009 Real 229166 Diff 0.54%
307200 Baud PS 4 DIV 0x0036 Real 305555 Diff 0.54%
PS 32 DIV 0x0007 Real 294642 Diff 4.09%
460800 Baud PS 4 DIV 0x0024 Real 458333 Diff 0.54%
PS 32 DIV 0x0005 Real 412500 Diff 10.48%
500000 Baud PS 4 DIV 0x0021 Real 500000 Diff 0.00%
PS 32 DIV 0x0004 Real 515625 Diff - 3.13%
576000 Baud PS 4 DIV 0x001d Real 568965 Diff 1.22%
PS 32 DIV 0x0004 Real 515625 Diff 10.48%
614400 Baud PS 4 DIV 0x001b Real 611111 Diff 0.54%
PS 32 DIV 0x0003 Real 687500 Diff -11.90%
921600 Baud PS 4 DIV 0x0012 Real 916666 Diff 0.54%
PS 32 DIV 0x0002 Real 1031250 Diff -11.90%
1000000 Baud PS 4 DIV 0x0011 Real 970588 Diff 2.94%
PS 32 DIV 0x0002 Real 1031250 Diff - 3.13%
1152000 Baud PS 4 DIV 0x000e Real 1178571 Diff - 2.31%
PS 32 DIV 0x0002 Real 1031250 Diff 10.48%
1500000 Baud PS 4 DIV 0x000b Real 1500000 Diff 0.00%
PS 32 DIV 0x0001 Real 2062500 Diff -37.50%
2000000 Baud PS 4 DIV 0x0008 Real 2062500 Diff - 3.13%
PS 32 DIV 0x0001 Real 2062500 Diff - 3.13%
2500000 Baud PS 4 DIV 0x0007 Real 2357142 Diff 5.71%
PS 32 DIV 0x0001 Real 2062500 Diff 17.50%
3000000 Baud PS 4 DIV 0x0006 Real 2750000 Diff 8.33%
PS 32 DIV 0x0001 Real 2062500 Diff 31.25%
3500000 Baud PS 4 DIV 0x0005 Real 3300000 Diff 5.71%
PS 32 DIV 0x0001 Real 2062500 Diff 41.07%
4000000 Baud PS 4 DIV 0x0004 Real 4125000 Diff - 3.13%
PS 32 DIV 0x0001 Real 2062500 Diff 48.44%
drivers/tty/serial/mpc52xx_uart.c | 5 +++--
1 files changed, 3 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/serial/mpc52xx_uart.c b/drivers/tty/serial/mpc52xx_uart.c
index 1093a88..5e2c6fe 100644
--- a/drivers/tty/serial/mpc52xx_uart.c
+++ b/drivers/tty/serial/mpc52xx_uart.c
@@ -262,8 +262,9 @@ static unsigned int mpc5200b_psc_set_baudrate(struct uart_port *port,
port->uartclk / 4);
divisor = (port->uartclk + 2 * baud) / (4 * baud);

- /* select the proper prescaler and set the divisor */
- if (divisor > 0xffff) {
+ /* select the proper prescaler and set the divisor
+ * prefer high prescaler for more tolerance on low baudrates */
+ if (divisor > 0xffff || baud <= 115200) {
divisor = (divisor + 4) / 8;
prescaler = 0xdd00; /* /32 */
} else
--
1.5.2.1



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