gpu/intel: Precisely compute the period exponent

Use the formula as specified in the intel drm header.
Signed-off-by: Corentin Noël <corentin.noel@collabora.com>
Reviewed-by: Antonio Caggiano <antonio.caggiano@collabora.com>

src/gpu/intel/intel_driver.cc

@@ -14,6 +14,7 @@
 #include <strings.h>
 #include <sys/ioctl.h>
 #include <unistd.h>
+#include <math.h>
 
 #include <i915_drm.h>
 #include <perf_data_reader.h>
@@ -282,31 +283,6 @@ bool IntelDriver::init_perfcnt()
     return true;
 }
 
-/// @return The OA exponent which essentially is a power of 2 of the timestamp frequency
-/// and it is used to tell the driver how often a report should be written to the stream.
-uint32_t oa_exponent_for_period(const uint64_t timestamp_frequency,
-    const uint64_t sampling_period_ns)
-{
-    constexpr uint32_t device_period_count = 32;
-    std::array<uint64_t, device_period_count> device_periods;
-
-    for (uint32_t i = 0; i < device_period_count; i++) {
-        device_periods[i] = 1000000000ull * (1u << i) / timestamp_frequency;
-    }
-
-    for (uint32_t i = 1; i < device_period_count; i++) {
-        if (sampling_period_ns >= device_periods[i - 1] && sampling_period_ns < device_periods[i]) {
-            if ((device_periods[i] - sampling_period_ns) >
-                (sampling_period_ns - device_periods[i - 1])) {
-                return i - 1;
-            }
-            return i;
-        }
-    }
-
-    return -1;
-}
-
 int IntelDriver::perf_open(const intel_perf_metric_set &metric_set,
     const uint64_t sampling_period_ns)
 {
@@ -324,7 +300,10 @@ int IntelDriver::perf_open(const intel_perf_metric_set &metric_set,
     properties[p++] = DRM_I915_PERF_PROP_OA_FORMAT;
     properties[p++] = metric_set.perf_oa_format;
 
-    auto oa_exponent = oa_exponent_for_period(timestamp_frequency, sampling_period_ns);
+    // The period_exponent gives a sampling period as follows:
+    // sample_period = timestamp_period * 2^(period_exponent + 1)
+    // where timestamp_period is 80ns for Haswell+
+    auto oa_exponent = (uint32_t) log2(sampling_period_ns * timestamp_frequency / 1000000000ull) - 1;
     properties[p++] = DRM_I915_PERF_PROP_OA_EXPONENT;
     properties[p++] = oa_exponent;