From 0cc93a460319294d085a5642b55050fadf39100e Mon Sep 17 00:00:00 2001
From: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Date: Wed, 15 Mar 2023 10:26:13 -0400
Subject: [PATCH] util/cpu: add big.LITTLE cpu detection
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it's useful for drivers to be able to detect big.LITTLE topology in
order to make decisions about how to handle workloads (specifically as
it relates to threading)

Reviewed-by: Rob Clark <robdclark@chromium.org>

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21926>
---
 src/util/u_cpu_detect.c | 35 +++++++++++++++++++++++++++++++++++
 src/util/u_cpu_detect.h | 11 +++++++++++
 2 files changed, 46 insertions(+)

diff --git a/src/util/u_cpu_detect.c b/src/util/u_cpu_detect.c
index 5a9baddfb87..2357c4f9780 100644
--- a/src/util/u_cpu_detect.c
+++ b/src/util/u_cpu_detect.c
@@ -38,6 +38,7 @@
 #include "util/u_debug.h"
 #include "u_cpu_detect.h"
 #include "u_math.h"
+#include "os_file.h"
 #include "c11/threads.h"
 
 #include <stdio.h>
@@ -478,6 +479,40 @@ get_cpu_topology(void)
 
    memset(util_cpu_caps.cpu_to_L3, 0xff, sizeof(util_cpu_caps.cpu_to_L3));
 
+#if DETECT_OS_LINUX
+   uint64_t big_cap = 0;
+   unsigned num_big_cpus = 0;
+   uint64_t *caps = malloc(sizeof(uint64_t) * util_cpu_caps.max_cpus);
+   bool fail = false;
+   for (unsigned i = 0; caps && i < util_cpu_caps.max_cpus; i++) {
+      char name[PATH_MAX];
+      snprintf(name, sizeof(name), "/sys/devices/system/cpu/cpu%u/cpu_capacity", i);
+      size_t size = 0;
+      char *cap = os_read_file(name, &size);
+      if (!cap) {
+         num_big_cpus = 0;
+         fail = true;
+         break;
+      }
+      errno = 0;
+      caps[i] = strtoull(cap, NULL, 10);
+      free(cap);
+      if (errno) {
+         fail = true;
+         break;
+      }
+      big_cap = MAX2(caps[i], big_cap);
+   }
+   if (!fail) {
+      for (unsigned i = 0; caps && i < util_cpu_caps.max_cpus; i++) {
+         if (caps[i] >= big_cap / 2)
+            num_big_cpus++;
+      }
+   }
+   free(caps);
+   util_cpu_caps.nr_big_cpus = num_big_cpus;
+#endif
+
 #if DETECT_ARCH_X86 || DETECT_ARCH_X86_64
    /* AMD Zen */
    if (util_cpu_caps.family >= CPU_AMD_ZEN1_ZEN2 &&
diff --git a/src/util/u_cpu_detect.h b/src/util/u_cpu_detect.h
index 0c59a008a6b..f9db16f82da 100644
--- a/src/util/u_cpu_detect.h
+++ b/src/util/u_cpu_detect.h
@@ -126,8 +126,19 @@ struct util_cpu_caps_t {
    unsigned max_vector_bits;
 
    uint16_t cpu_to_L3[UTIL_MAX_CPUS];
+
    /* Affinity masks for each L3 cache. */
    util_affinity_mask *L3_affinity_mask;
+   /**
+    * number of "big" CPUs in big.LITTLE configuration
+    * 
+    * a "big" CPU is defined as anything with >= 50% the capacity of the largest CPU,
+    * useful for drivers determining how many and what kinds of threads to use
+    * example: 1x prime + 3x big + 4x little = 4x "big" cores
+    * 
+    * A value of zero indicates that CPUs are homogeneous.
+    */
+   int16_t nr_big_cpus;
 };
 
 struct _util_cpu_caps_state_t {