drm: amd: amdgpu: ACPI: Add comment about ACPI_FADT_LOW_POWER_S0

According to the ACPI specification [1], the ACPI_FADT_LOW_POWER_S0
flag merely means that it is better to use low-power S0 idle on the
given platform than S3 (provided that the latter is supported) and it
doesn't preclude using either of them (which of them will be used
depends on the choices made by user space).

However, on some systems that flag is used to indicate whether or not
to enable special firmware mechanics allowing the system to save more
energy when suspended to idle.  If that flag is unset, doing so is
generally risky.

Accordingly, add a comment to explain the ACPI_FADT_LOW_POWER_S0 check
in amdgpu_acpi_is_s0ix_active(), the purpose of which is otherwise
somewhat unclear.

Link: https://uefi.org/specs/ACPI/6.4/05_ACPI_Software_Programming_Model/ACPI_Software_Programming_Model.html#fixed-acpi-description-table-fadt # [1]
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c

@@ -1066,6 +1066,12 @@ bool amdgpu_acpi_is_s0ix_active(struct amdgpu_device *adev)
 	    (pm_suspend_target_state != PM_SUSPEND_TO_IDLE))
 		return false;
 
+	/*
+	 * If ACPI_FADT_LOW_POWER_S0 is not set in the FADT, it is generally
+	 * risky to do any special firmware-related preparations for entering
+	 * S0ix even though the system is suspending to idle, so return false
+	 * in that case.
+	 */
 	if (!(acpi_gbl_FADT.flags & ACPI_FADT_LOW_POWER_S0)) {
 		dev_warn_once(adev->dev,
 			      "Power consumption will be higher as BIOS has not been configured for suspend-to-idle.\n"