[PATCH v1 0/2] spi: fsl-dspi: better approximation for baudrate

[Andy Shevchenko <andy.shevchenko@xxxxxxxxx>]

This series makes better approximation when baud rate divisor parameters are
calculated.

The algorithm is represent as Python script here [1]. First parameter is
algorithm (0 - original, 1 - after patch 1/2, 2 - after patch 2/2, 3 - memory
vs. performance: use precalculated scales).

I played with let's say standard baud rates (which I used for Quark) and
separately run algorithms 0 and 1 for range 100-5000 baud. Input frequency is
64MHz. It seems my algo shows better results in all cases. Here is a diff for
'standard' baud rates
(speed_hz, DBR, i, BR, j, PBR, real baud rate, difference):

--- result0     2015-04-02 00:38:48.845761004 +0300
+++ result1     2015-04-02 00:38:48.913760154 +0300
@@ -5,7 +5,7 @@
 25000000 0 0  2     0 2 16000000 9000000
 20000000 0 0  2     0 2 16000000 4000000
 16667000 0 0  2     0 2 16000000 667000
-13333000 0 0  2     0 2 16000000 2667000
+13333000 0 0  2     1 3 10666666 2666334
 12500000 0 0  2     1 3 10666666 1833334
 10000000 0 0  2     1 3 10666666 666666
 8000000  0 1  4     0 2 8000000 0
@@ -17,36 +17,36 @@
 3140500  0 1  4     2 5 3200000 59500
 3125000  0 1  4     2 5 3200000 75000
 3109500  0 1  4     2 5 3200000 90500
-2500000  0 1  4     3 7 2285714 214286
+2500000  0 3  8     1 3 2666666 166666
 2000000  0 4  16    0 2 2000000 0
 1563000  0 3  8     2 5 1600000 37000
-1250000  0 3  8     3 7 1142857 107143
+1250000  0 4  16    1 3 1333333 83333
 1006000  0 5  32    0 2 1000000 6000
 1000000  0 5  32    0 2 1000000 0
 994000   0 5  32    0 2 1000000 6000
 800000   0 4  16    2 5 800000 0
-781250   0 5  32    1 3 666666 114584
-625000   0 4  16    3 7 571428 53572
+781250   0 4  16    2 5 800000 18750
+625000   0 5  32    1 3 666666 41666
 500000   0 6  64    0 2 500000 0
 400000   0 5  32    2 5 400000 0
-390625   0 6  64    1 3 333333 57292
+390625   0 5  32    2 5 400000 9375
 250000   0 7  128   0 2 250000 0
 200000   0 6  64    2 5 200000 0
 205313   0 6  64    2 5 200000 5313
-195313   0 7  128   1 3 166666 28647
-185313   0 7  128   1 3 166666 18647
+195313   0 6  64    2 5 200000 4687
+185313   0 6  64    2 5 200000 14687
 125000   0 8  256   0 2 125000 0
 100700   0 7  128   2 5 100000 700
 100000   0 7  128   2 5 100000 0
-99300    0 8  256   1 3 83333 15967
+99300    0 7  128   2 5 100000 700
 50000    0 8  256   2 5 50000 0
 25000    0 9  512   2 5 25000 0
-10066    0 10 1024  3 7 8928 1138
-10016    0 10 1024  3 7 8928 1088
-9966     0 10 1024  3 7 8928 1038
-5065     0 11 2048  3 7 4464 601
-5040     0 11 2048  3 7 4464 576
-5015     0 11 2048  3 7 4464 551
+10066    0 11 2048  1 3 10416 350
+10016    0 11 2048  1 3 10416 400
+9966     0 11 2048  1 3 10416 450
+5065     0 12 4096  1 3 5208 143
+5040     0 12 4096  1 3 5208 168
+5015     0 12 4096  1 3 5208 193
 1007     0 15 32768 0 2 976 31
 1002     0 15 32768 0 2 976 26
 997      0 15 32768 0 2 976 21

Patch 2 is RFC, since I don't know if all DSPI modules has that bit (at least
what I found on public for MC55xx) and how duty cycle is significant.

Not compiled. Not tested.

[1] http://pastebin.com/uy0CKc6b

Andy Shevchenko (2):
  spi: fsl-dspi: increase precision of baud rate approximation
  spi: fsl-dspi: use double baud rate in approximation

 drivers/spi/spi-fsl-dspi.c | 50 +++++++++++++++++++++-------------------------
 1 file changed, 23 insertions(+), 27 deletions(-)

-- 
1.9.3

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