* [PATCH] spi: mvebu: fix baudrate calculation for armada variants
@ 2016-12-08 8:21 Uwe Kleine-König
0 siblings, 0 replies; only message in thread
From: Uwe Kleine-König @ 2016-12-08 8:21 UTC (permalink / raw)
To: Mark Brown, Greg Ungerer
Cc: linux-spi-u79uwXL29TY76Z2rM5mHXA, Ezequiel Garcia,
kernel-bIcnvbaLZ9MEGnE8C9+IrQ
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: df59fa7f4bca ("spi: orion: support armada extended baud rates")
Signed-off-by: Uwe Kleine-König <uwe-rXY34ruvC2xidJT2blvkqNi2O/JbrIOy@public.gmane.org>
---
drivers/spi/spi-orion.c | 83 ++++++++++++++++++++++++++++++++-----------------
1 file changed, 54 insertions(+), 29 deletions(-)
diff --git a/drivers/spi/spi-orion.c b/drivers/spi/spi-orion.c
index ded37025b445..af0cad18c6cd 100644
--- a/drivers/spi/spi-orion.c
+++ b/drivers/spi/spi-orion.c
@@ -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;
-
- best_err = speed;
- best_spr = 0;
- best_sppr = 0;
-
- /* Iterate over the valid range looking for best fit */
- for (sppr = 0; sppr < 8; sppr++) {
- sppr2 = 0x1 << sppr;
-
- spr = tclk_hz / sppr2;
- spr = DIV_ROUND_UP(spr, speed);
- if ((spr == 0) || (spr > 15))
- continue;
-
- clk = tclk_hz / (spr * sppr2);
- err = speed - clk;
-
- if (err < best_err) {
- best_spr = spr;
- best_sppr = sppr;
- best_err = 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.
+ */
- if ((best_sppr == 0) && (best_spr == 0))
- return -EINVAL;
+ /* best integer divider: */
+ unsigned divider = DIV_ROUND_UP(tclk_hz, speed);
+ unsigned spr, sppr;
+
+ if (divider < 16) {
+ /* This is the easy case, divider is less than 16 */
+ spr = divider;
+ sppr = 0;
+
+ } 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;
+
+ /*
+ * 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;
+
+ /*
+ * 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 sspr to fit into 3 bits:
+ */
+ if (sspr > 7)
+ 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
--
2.10.2
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