[PATCH i-g-t 6/7] lib/igt_pm: Add drm_kms_helper.poll save/restore helpers

[Pranay Samala <pranay.samala@intel.com>]

The new PME hotplug subtest needs to save the drm_kms_helper.poll module
parameter and put it back on exit, so that an earlier test cannot leave
polling disabled and make the measurement pass for the wrong reason.

tests/intel/kms_pm_dc.c and tests/kms_prime.c already open code exactly
that, each with its own copy of the KMS_HELPER and KMS_POLL_DISABLE
defines and its own file scope kms_poll_saved_state global. Rather than
add a third copy, add igt_pm_kms_poll_save(), _set() and _restore().
Converting those two existing callers is left to a separate patch.

Save and set stay separate rather than becoming one "save and disable"
helper, because a caller may want to record the original value and put it
back on exit while leaving polling switched on.

igt_pm_kms_poll_save() installs the exit handler itself, and
igt_pm_kms_poll_restore() does nothing unless a save actually happened, so
a restore can never run without a matching save.

Assisted-by: GitHub_Copilot:claude-opus-5
Signed-off-by: Pranay Samala <pranay.samala@intel.com>
---
 lib/igt_pm.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 lib/igt_pm.h |  3 ++
 2 files changed, 93 insertions(+)

diff --git a/lib/igt_pm.c b/lib/igt_pm.c
index a9fd9610a..f6a99d233 100644
--- a/lib/igt_pm.c
+++ b/lib/igt_pm.c
@@ -1679,6 +1679,96 @@ uint64_t igt_pm_get_wakeup_active_count(struct pci_device *pci_dev)
 	return igt_pm_read_power_attr_u64(pci_dev, "wakeup_active_count");
 }
 
+#define IGT_KMS_HELPER_PARAMS "/sys/module/drm_kms_helper/parameters"
+
+static struct {
+	bool saved;
+	bool orig_poll;
+} __igt_pm_kms_poll;
+
+static void __igt_pm_kms_poll_exit_handler(int sig)
+{
+	igt_pm_kms_poll_restore();
+}
+
+/**
+ * igt_pm_kms_poll_save:
+ *
+ * Saves the current drm_kms_helper.poll value and installs an exit handler to
+ * restore it. Safe to call more than once, only the first value is kept.
+ *
+ * Returns: false if the module parameter is not available, in which case
+ * nothing was saved.
+ */
+bool igt_pm_kms_poll_save(void)
+{
+	int dir;
+
+	if (__igt_pm_kms_poll.saved)
+		return true;
+
+	dir = open(IGT_KMS_HELPER_PARAMS, O_RDONLY);
+	if (dir < 0)
+		return false;
+
+	if (!__igt_sysfs_get_boolean(dir, "poll", &__igt_pm_kms_poll.orig_poll)) {
+		close(dir);
+		return false;
+	}
+	close(dir);
+
+	__igt_pm_kms_poll.saved = true;
+	igt_install_exit_handler(__igt_pm_kms_poll_exit_handler);
+
+	igt_debug("Saved drm_kms_helper.poll = %s\n",
+		  __igt_pm_kms_poll.orig_poll ? "Y" : "N");
+
+	return true;
+}
+
+/**
+ * igt_pm_kms_poll_set:
+ * @enable: whether to enable connector polling
+ *
+ * Writes the drm_kms_helper.poll module parameter. Callers are expected to have
+ * called igt_pm_kms_poll_save() first so the original value is restored on exit.
+ */
+void igt_pm_kms_poll_set(bool enable)
+{
+	int dir;
+
+	dir = open(IGT_KMS_HELPER_PARAMS, O_RDONLY);
+	igt_assert_f(dir >= 0, "Can't open %s\n", IGT_KMS_HELPER_PARAMS);
+
+	igt_sysfs_set_boolean(dir, "poll", enable);
+	close(dir);
+
+	igt_debug("drm_kms_helper.poll = %s\n", enable ? "Y" : "N");
+}
+
+/**
+ * igt_pm_kms_poll_restore:
+ *
+ * Restores drm_kms_helper.poll to the value saved by
+ * igt_pm_kms_poll_save(). Does nothing if it was never saved, so it is safe to
+ * use as an exit handler that may run before any save happened.
+ */
+void igt_pm_kms_poll_restore(void)
+{
+	int dir;
+
+	if (!__igt_pm_kms_poll.saved)
+		return;
+
+	dir = open(IGT_KMS_HELPER_PARAMS, O_RDONLY);
+	if (dir >= 0) {
+		__igt_sysfs_set_boolean(dir, "poll", __igt_pm_kms_poll.orig_poll);
+		close(dir);
+	}
+
+	__igt_pm_kms_poll.saved = false;
+}
+
 /**
  * igt_pm_ignore_slpc_efficient_freq:
  * @i915: open i915 drm file descriptor
diff --git a/lib/igt_pm.h b/lib/igt_pm.h
index eba54efa2..307fd9e34 100644
--- a/lib/igt_pm.h
+++ b/lib/igt_pm.h
@@ -107,6 +107,9 @@ bool igt_pm_has_wakeup_support(struct pci_device *pci_dev);
 void igt_pm_set_wakeup_enabled(struct pci_device *pci_dev, bool enable);
 void igt_pm_restore_wakeup(void);
 uint64_t igt_pm_get_wakeup_active_count(struct pci_device *pci_dev);
+bool igt_pm_kms_poll_save(void);
+void igt_pm_kms_poll_set(bool enable);
+void igt_pm_kms_poll_restore(void);
 void igt_pm_ignore_slpc_efficient_freq(int i915, int gtfd, bool val);
 bool igt_has_pci_pm_capability(struct pci_device *pci_dev);
 bool igt_pm_pci_pme_supported(struct pci_device *pci_dev,
-- 
2.53.0