acpi-cpufreq: Add comment under ACPI_ADR_SPACE_SYSTEM_IO case

policy->cur is now set by cpufreq core when cpufreq_driver->get() is defined and
so drivers aren't required to set it. When space_id is ACPI_ADR_SPACE_SYSTEM_IO
for acpi cpufreq driver it doesn't set ->get to a valid function pointer and so
policy->cur is required to be set by driver.

This is already followed in acpi-cpufreq driver. This patch adds a comment
describing why we need to set policy->cur from driver.

Suggested-by: Rafael J. Wysocki <[e-mail]>
Signed-off-by: Viresh Kumar <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/cpufreq/acpi-cpufreq.c

···
 
 	switch (perf->control_register.space_id) {
 	case ACPI_ADR_SPACE_SYSTEM_IO:
-		/* Current speed is unknown and not detectable by IO port */
+		/*
+		 * The core will not set policy->cur, because
+		 * cpufreq_driver->get is NULL, so we need to set it here.
+		 * However, we have to guess it, because the current speed is
+		 * unknown and not detectable via IO ports.
+		 */
 		policy->cur = acpi_cpufreq_guess_freq(data, policy->cpu);
 		break;
 	case ACPI_ADR_SPACE_FIXED_HARDWARE: