[CPUFREQ] pcc driver should check for pcch method before calling _OSC

The pcc specification documents an _OSC method that's incompatible with the
one defined as part of the ACPI spec. This shouldn't be a problem as both
are supposed to be guarded with a UUID. Unfortunately approximately nobody
(including HP, who wrote this spec) properly check the UUID on entry to the
_OSC call. Right now this could result in surprising behaviour if the pcc
driver performs an _OSC call on a machine that doesn't implement the pcc
specification. Check whether the PCCH method exists first in order to reduce
this probability.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Naga Chumbalkar <nagananda.chumbalkar@hp.com>
Signed-off-by: Dave Jones <davej@redhat.com>

diff --git a/arch/x86/kernel/cpu/cpufreq/pcc-cpufreq.c b/arch/x86/kernel/cpu/cpufreq/pcc-cpufreq.c
index ce7cde7..01bd25c 100644 (file)
--- a/arch/x86/kernel/cpu/cpufreq/pcc-cpufreq.c
+++ b/arch/x86/kernel/cpu/cpufreq/pcc-cpufreq.c
@@ -397,13 +397,17 @@ static int __init pcc_cpufreq_probe(void)
        struct pcc_memory_resource *mem_resource;
        struct pcc_register_resource *reg_resource;
        union acpi_object *out_obj, *member;
-       acpi_handle handle, osc_handle;
+       acpi_handle handle, osc_handle, pcch_handle;
        int ret = 0;
 
        status = acpi_get_handle(NULL, "\\_SB", &handle);
        if (ACPI_FAILURE(status))
                return -ENODEV;
 
+       status = acpi_get_handle(handle, "PCCH", &pcch_handle);
+       if (ACPI_FAILURE(status))
+               return -ENODEV;
+
        status = acpi_get_handle(handle, "_OSC", &osc_handle);
        if (ACPI_SUCCESS(status)) {
                ret = pcc_cpufreq_do_osc(&osc_handle);