Re: [PATCH] return error when failing to set minfreq

[Dominik Brodowski <linux@dominikbrodowski.net>]

On Mon, Jun 05, 2006 at 08:31:48PM +0200, Mattia Dongili wrote:
> Hello,
> 
> no comment on the following?
> 
> On Fri, May 26, 2006 at 12:34:04AM +0200, Mattia Dongili wrote:
> > Hello,
> > 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).
> > 
> > I'm unsure if things could be improved here.  The offending instruction
> > is in cpufreq_verify_within_limits (cpufreq.h:241):
> > 
> > 	if (policy->min > policy->max)
> >                 policy->min = policy->max;
> > 
> > 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@dominikbrodowski.net>

Thanks,
	Dominik