[PATCH i-g-t v3 1/2] lib/igt_pm: Add CPU idle state control for vblank timing stability

[Jason-JH Lin <jason-jh.lin@mediatek.com>]

Add igt_pm_disable_cpu_deep_sleep() and igt_pm_enable_cpu_deep_sleep()
APIs to control deep CPU idle states (C-states) via sysfs. This prevents
IRQ latency spikes that affect vblank timestamp acquisition.

The functions control state1 and deeper states on all CPUs while keeping
state0 (WFI) unchanged, providing a balance between timing accuracy and
power efficiency.

Signed-off-by: Jason-JH Lin <jason-jh.lin@mediatek.com>
---
 lib/igt_pm.c | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 lib/igt_pm.h |  2 ++
 2 files changed, 77 insertions(+)

diff --git a/lib/igt_pm.c b/lib/igt_pm.c
index 64b15dd08302..7a8ad8ae3dea 100644
--- a/lib/igt_pm.c
+++ b/lib/igt_pm.c
@@ -1713,3 +1713,78 @@ unsigned int igt_read_pkgc_counter(int debugfs_root_fd)
 
 	return igt_get_dc_counter(str);
 }
+
+static bool igt_pm_cpu_deep_sleep_control(bool disable)
+{
+	char cpu_path[256];
+	char state_path[512];
+	int cpu, state;
+	int count = 0;
+	FILE *fp;
+	const char *value = disable ? "1" : "0";
+	const char *action = disable ? "Disabled" : "Restored";
+
+	/* Control state1 and deeper for all CPUs */
+	for (cpu = 0; cpu < 256; cpu++) {
+		snprintf(cpu_path, sizeof(cpu_path),
+			 "/sys/devices/system/cpu/cpu%d", cpu);
+
+		/* Check if CPU exists */
+		if (access(cpu_path, F_OK) != 0)
+			break;
+
+		/* Control state1 and deeper (keep state0 WFI unchanged) */
+		for (state = 1; state < 10; state++) {
+			snprintf(state_path, sizeof(state_path),
+				 "%s/cpuidle/state%d/disable", cpu_path, state);
+
+			if (access(state_path, W_OK) != 0)
+				break;
+
+			/* Write '1' to disable, '0' to enable */
+			fp = fopen(state_path, "w");
+			if (fp) {
+				fprintf(fp, "%s", value);
+				fclose(fp);
+				count++;
+			}
+		}
+	}
+
+	if (count > 0)
+		igt_info("%s deep CPU idle states for %d CPU/state combinations\n",
+			 action, count);
+	else
+		igt_warn("No CPU idle states found to control\n");
+
+	return count > 0;
+}
+
+/**
+ * igt_pm_disable_cpu_deep_sleep:
+ *
+ * Disables deep CPU idle states (C-states) to prevent IRQ latency spikes
+ * during vblank events. This is a system-wide operation that affects all CPUs.
+ *
+ * Disables state1 and deeper states while keeping state0 (WFI) unchanged
+ * to ensure stable vblank timing.
+ *
+ * Returns: true on success, false on failure
+ */
+bool igt_pm_disable_cpu_deep_sleep(void)
+{
+	return igt_pm_cpu_deep_sleep_control(true);
+}
+
+/**
+ * igt_pm_enable_cpu_deep_sleep:
+ *
+ * Re-enables deep CPU idle states (C-states) that were previously disabled
+ * by igt_pm_disable_cpu_deep_sleep(). This restores power-saving idle states.
+ *
+ * Returns: true on success, false on failure
+ */
+bool igt_pm_enable_cpu_deep_sleep(void)
+{
+	return igt_pm_cpu_deep_sleep_control(false);
+}
diff --git a/lib/igt_pm.h b/lib/igt_pm.h
index 6a33c6db22fb..c0e35dbe721e 100644
--- a/lib/igt_pm.h
+++ b/lib/igt_pm.h
@@ -114,5 +114,7 @@ bool igt_dc_state_wait_entry(int debugfs_fd, int dc_flag, int prev_dc_count);
 const char *igt_dc_state_name(int dc_flag);
 void igt_require_dc_counter(int debugfs_fd, int dc_flag);
 unsigned int igt_read_pkgc_counter(int debugfs_root_fd);
+bool igt_pm_disable_cpu_deep_sleep(void);
+bool igt_pm_enable_cpu_deep_sleep(void);
 
 #endif /* IGT_PM_H */
-- 
2.43.0