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spi: mvebu: fix baudrate calculation for armada variant

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The calculation of SPR and SPPR doesn't round correctly at several
places which might result in baud rates that are too big. For example
with tclk_hz = 250000001 and target rate 25000000 it determined a
divider of 10 which is wrong.

Instead of fixing all the corner cases replace the calculation by an
algorithm without a loop which should even be quicker to execute apart
from being correct.

Fixes: df59fa7f4b ("spi: orion: support armada extended baud rates")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
This commit is contained in:
Uwe Kleine-König 2016-12-08 17:37:08 +01:00 committed by Mark Brown
parent df34d04a6f
commit 7243e0b207
1 changed files with 49 additions and 24 deletions
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73
drivers/spi/spi-orion.c
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@ -138,37 +138,62 @@ static int orion_spi_baudrate_set(struct spi_device *spi, unsigned int speed)
tclk_hz = clk_get_rate(orion_spi->clk);
if (devdata->typ == ARMADA_SPI) {
unsigned int clk, spr, sppr, sppr2, err;
unsigned int best_spr, best_sppr, best_err;
/*
* Given the core_clk (tclk_hz) and the target rate (speed) we
* determine the best values for SPR (in [0 .. 15]) and SPPR (in
* [0..7]) such that
*
* core_clk / (SPR * 2 ** SPPR)
*
* is as big as possible but not bigger than speed.
*/
best_err = speed;
best_spr = 0;
best_sppr = 0;
/* best integer divider: */
unsigned divider = DIV_ROUND_UP(tclk_hz, speed);
unsigned spr, sppr;
/* Iterate over the valid range looking for best fit */
for (sppr = 0; sppr < 8; sppr++) {
sppr2 = 0x1 << sppr;
if (divider < 16) {
/* This is the easy case, divider is less than 16 */
spr = divider;
sppr = 0;
spr = tclk_hz / sppr2;
spr = DIV_ROUND_UP(spr, speed);
if ((spr == 0) || (spr > 15))
continue;
} else {
unsigned two_pow_sppr;
/*
* Find the highest bit set in divider. This and the
* three next bits define SPR (apart from rounding).
* SPPR is then the number of zero bits that must be
* appended:
*/
sppr = fls(divider) - 4;
clk = tclk_hz / (spr * sppr2);
err = speed - clk;
/*
* As SPR only has 4 bits, we have to round divider up
* to the next multiple of 2 ** sppr.
*/
two_pow_sppr = 1 << sppr;
divider = (divider + two_pow_sppr - 1) & -two_pow_sppr;
if (err < best_err) {
best_spr = spr;
best_sppr = sppr;
best_err = err;
}
/*
* recalculate sppr as rounding up divider might have
* increased it enough to change the position of the
* highest set bit. In this case the bit that now
* doesn't make it into SPR is 0, so there is no need to
* round again.
*/
sppr = fls(divider) - 4;
spr = divider >> sppr;
/*
* Now do range checking. SPR is constructed to have a
* width of 4 bits, so this is fine for sure. So we
* still need to check for sppr to fit into 3 bits:
*/
if (sppr > 7)
return -EINVAL;
}
if ((best_sppr == 0) && (best_spr == 0))
return -EINVAL;
prescale = ((best_sppr & 0x6) << 5) |
((best_sppr & 0x1) << 4) | best_spr;
prescale = ((sppr & 0x6) << 5) | ((sppr & 0x1) << 4) | spr;
} else {
/*
* the supported rates are: 4,6,8...30