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[CPUFREQ] return error when failing to set minfreq

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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>

--
This commit is contained in:
Mattia Dongili 2006-07-05 23:12:20 +02:00 committed by Dave Jones
parent 49b1e3ea19
commit 9c9a43ed27
2 changed files with 9 additions and 1 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

3
Documentation/cpu-freq/user-guide.txt
View file

@ -157,6 +157,9 @@ scaling_min_freq and
scaling_max_freq show the current "policy limits" (in
kHz). By echoing new values into these
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

5
drivers/cpufreq/cpufreq.c
View file

@ -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));
if (policy->min > data->min && policy->min > policy->max) {
ret = -EINVAL;
goto error_out;
}
/* verify the cpu speed can be set within this limit */
ret = cpufreq_driver->verify(policy);
if (ret)