From: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
To: rjw@rjwysocki.net, lenb@kernel.org
Cc: linux-pm@vger.kernel.org,
Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Subject: [PATCH] cpufreq: intel_pstate: Increase precision
Date: Mon, 5 Jun 2017 18:01:04 -0700 [thread overview]
Message-ID: <1496710864-117662-1-git-send-email-srinivas.pandruvada@linux.intel.com> (raw)
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.
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
---
drivers/cpufreq/intel_pstate.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
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)
--
2.7.4
reply other threads:[~2017-06-06 1:02 UTC|newest]
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