Commit 9c9a43ed authored Jul 05, 2006 by Mattia Dongili Committed by Dave Jones Jul 31, 2006

[CPUFREQ] return error when failing to set minfreq



I just stumbled on this bug/feature, this is how to reproduce it:

# echo 450000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
# echo 450000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
# echo powersave > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
# cpufreq-info -p
450000 450000 powersave
# echo 1800000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq ; echo $?
0
# cpufreq-info -p
450000 450000 powersave

Here it is. The kernel refuses to set a min_freq higher than the
max_freq but it allows a max_freq lower than min_freq (lowering min_freq
also).

This behaviour is pretty straightforward (but undocumented) and it
doesn't return an error altough failing to accomplish the requested
action (set min_freq).
The problem (IMO) is basically that userspace is not allowed to set a
full policy atomically while the kernel always does that thus it must
enforce an ordering on operations.

The attached patch returns -EINVAL if trying to increase frequencies
starting from scaling_min_freq and documents the correct ordering of writes.

Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Dominik Brodowski <linux at dominikbrodowski.net>
Signed-off-by: Dave Jones <davej@redhat.com>

--

parent 49b1e3ea

Documentation/cpu-freq/user-guide.txt

+4 −1

Original line number	Original line	Diff line number	Diff line
	@@ -157,6 +157,9 @@ scaling_min_freq and
	scaling_max_freq show the current "policy limits" (in		scaling_max_freq show the current "policy limits" (in
	kHz). By echoing new values into these		kHz). By echoing new values into these
	files, you can change these limits.		files, you can change these limits.
			NOTE: when setting a policy you need to
			first set scaling_max_freq, then
			scaling_min_freq.


	If you have selected the "userspace" governor which allows you to		If you have selected the "userspace" governor which allows you to

drivers/cpufreq/cpufreq.c

+5 −0

Original line number	Original line	Diff line number	Diff line
	@@ -1343,6 +1343,11 @@ static int __cpufreq_set_policy(struct cpufreq_policy *data, struct cpufreq_poli

	memcpy(&policy->cpuinfo, &data->cpuinfo, sizeof(struct cpufreq_cpuinfo));		memcpy(&policy->cpuinfo, &data->cpuinfo, sizeof(struct cpufreq_cpuinfo));

			if (policy->min > data->min && policy->min > policy->max) {
			ret = -EINVAL;
			goto error_out;
			}

	/* verify the cpu speed can be set within this limit */		/* verify the cpu speed can be set within this limit */
	ret = cpufreq_driver->verify(policy);		ret = cpufreq_driver->verify(policy);
	if (ret)		if (ret)