# cpufreq: intel_pstate: Increase precision

## Commit Message

Srinivas Pandruvada June 6, 2017, 1:01 a.m. UTC
```In some cases the scaling max/min limit set via cpufreq interface doesn't
result in correct max/min.

For example, with the data below:

cpuinfo_max_freq:3700000
scaling_max_freq:2500000
HWP max ratio = 37

With the current fixed point conversion to ratio using 14 bit shift:
max_perf = (2500000 << 14) / 3700000 = 11070
Rounding up with 14 = 11070
HWP max ratio corresponding to 2500000 will be
= (hwp max ratio * max_perf) >> 14
= 24

So this will result in 100Mhz less frequency than what is requested,
with scaling factor of 100000.

To fix this if we increase the shift to 15 bits.
max_perf = (2500000 << 15) / 3700000 = 22140
rounded up with 15 = 22144
The new max ratio corresponding to 2500000 will be
= (hwp max ratio * max_perf) >> 15
= 25
This will result in correct scaling max frequency.

This patch changes EXT_BITS to 7 from 6, so this will result in 15 bit
shift during fixed point math above.

---
drivers/cpufreq/intel_pstate.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
```

## Patch

```diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 640eb7e4..6386422 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -52,7 +52,7 @@
#define int_tofp(X) ((int64_t)(X) << FRAC_BITS)
#define fp_toint(X) ((X) >> FRAC_BITS)

-#define EXT_BITS 6
+#define EXT_BITS 7
#define EXT_FRAC_BITS (EXT_BITS + FRAC_BITS)
#define fp_ext_toint(X) ((X) >> EXT_FRAC_BITS)
#define int_ext_tofp(X) ((int64_t)(X) << EXT_FRAC_BITS)

```