[PATCH i-g-t] lib/igt_pm: Skip LPM policy control on unsupported SATA hosts

[Chaitanya Kumar Borah]

With [1], LPM policy control is now disabled for external SATA ports.
This led to a regression in our CI runs [2]. Discussions on the regression
resulted in patch [3] which introduces a new sysfs entry, querying
which we can determine if LPM policy control is allowed on a host or not.

Store/Re-store LPM policy only when it is allowed. If the sysfs entry is
not present assume LPM policy control is allowed. This  maintains backward
compatibility.

[1] https://lore.kernel.org/linux-ide/20250701125321.69496-8-dlemoal@kernel.org/
[2] https://lore.kernel.org/linux-ide/07563042-6576-41cd-9a95-de83cfc95de1@intel.com/
[3] https://lore.kernel.org/linux-ide/20250730001947.332661-1-dlemoal@kernel.org/

Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah at intel.com>
---
 lib/igt_pm.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/lib/igt_pm.c b/lib/igt_pm.c
index cf1c442f8..1ffcdcef3 100644
--- a/lib/igt_pm.c
+++ b/lib/igt_pm.c
@@ -321,6 +321,35 @@ void igt_pm_enable_audio_runtime_pm(void)
 		igt_debug("Failed to enable audio runtime PM! (%d)\n", -err);
 }
 
+static bool lpm_supported_by_sata_host(int host_num)
+{
+	int fd;
+	ssize_t len;
+	char buf[2];
+	char file_name[PATH_MAX];
+
+	snprintf(file_name, PATH_MAX,
+		 "/sys/class/scsi_host/host%d/link_power_management_supported",
+		 host_num);
+
+	fd = open(file_name, O_RDONLY);
+
+	/* assume LPM is supported if sysfs entry is absent. This preserves default behaviour */
+	if (fd < 0)
+		return true;
+
+	len = read(fd, buf, 1);
+
+	close(fd);
+
+	if (len <= 0)
+		return true;
+
+	buf[len] = '\0';
+
+	return buf[0] == '1';
+}
+
 static void __igt_pm_enable_sata_link_power_management(void)
 {
 	int fd, i;
@@ -372,6 +401,9 @@ static void __igt_pm_enable_sata_link_power_management(void)
 	igt_install_exit_handler(__igt_pm_sata_link_pm_exit_handler);
 
 	for (i = 0; i < __scsi_host_cnt; i++) {
+		if (!lpm_supported_by_sata_host(i))
+			continue;
+
 		snprintf(file_name, PATH_MAX,
 			 "/sys/class/scsi_host/host%d/link_power_management_policy",
 			 i);
@@ -412,6 +444,9 @@ static void __igt_pm_restore_sata_link_power_management(void)
 	for (i = 0; i < __scsi_host_cnt; i++) {
 		int8_t policy;
 
+		if (!lpm_supported_by_sata_host(i))
+			continue;
+
 		if (__sata_pm_policies[i] == POLICY_UNKNOWN)
 			continue;
 		else
-- 
2.25.1