linux-kernel - Re: [PATCH 2/2] cpufreq: CPPC: Fix error handling in cppc_scale_freq

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Message-ID: <aIs6d4ebRKkbz0az@arm.com>
Date: Thu, 31 Jul 2025 11:42:37 +0200
From: Beata Michalska <beata.michalska@....com>
To: Jie Zhan <zhanjie9@...ilicon.com>
Cc: Bowen Yu <yubowen8@...wei.com>, rafael@...nel.org,
	viresh.kumar@...aro.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linuxarm@...wei.com,
	jonathan.cameron@...wei.com, lihuisong@...wei.com,
	zhenglifeng1@...wei.com
Subject: Re: [PATCH 2/2] cpufreq: CPPC: Fix error handling in
 cppc_scale_freq_workfn()

On Thu, Jul 31, 2025 at 04:52:05PM +0800, Jie Zhan wrote:
> 
> 
> On 31/07/2025 16:19, Beata Michalska wrote:
> > Hi Bowen, Jie
> > On Wed, Jul 30, 2025 at 11:23:12AM +0800, Bowen Yu wrote:
> >> From: Jie Zhan <zhanjie9@...ilicon.com>
> >>
> >> Perf counters could be 0 if the cpu is in a low-power idle state. Just try
> >> it again next time and update the frequency scale when the cpu is active
> >> and perf counters successfully return.
> >>
> >> Also, remove the FIE source on an actual failure.
> >>
> >> Signed-off-by: Jie Zhan <zhanjie9@...ilicon.com>
> >> ---
> >>  drivers/cpufreq/cppc_cpufreq.c | 13 ++++++++++++-
> >>  1 file changed, 12 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> >> index 904006027df2..e95844d3d366 100644
> >> --- a/drivers/cpufreq/cppc_cpufreq.c
> >> +++ b/drivers/cpufreq/cppc_cpufreq.c
> >> @@ -78,12 +78,23 @@ static void cppc_scale_freq_workfn(struct kthread_work *work)
> >>  	struct cppc_cpudata *cpu_data;
> >>  	unsigned long local_freq_scale;
> >>  	u64 perf;
> >> +	int ret;
> >>  
> >>  	cppc_fi = container_of(work, struct cppc_freq_invariance, work);
> >>  	cpu_data = cppc_fi->cpu_data;
> >>  
> >> -	if (cppc_get_perf_ctrs(cppc_fi->cpu, &fb_ctrs)) {
> >> +	ret = cppc_get_perf_ctrs(cppc_fi->cpu, &fb_ctrs);
> >> +	/*
> >> +	 * Perf counters could be 0 if the cpu is in a low-power idle state.
> >> +	 * Just try it again next time.
> >> +	 */
> >> +	if (ret == -EFAULT)
> >> +		return;
> > Which counters are we actually talking about here ?
> 
> Delivered performance counter and reference performance counter.
> They are actually AMU CPU_CYCLES and CNT_CYCLES event counters.
That does track then.
> 
> >> +
> >> +	if (ret) {
> >>  		pr_warn("%s: failed to read perf counters\n", __func__);
> >> +		topology_clear_scale_freq_source(SCALE_FREQ_SOURCE_CPPC,
> >> +						 cpu_data->shared_cpu_map);
> >>  		return;
> >>  	}
> > And the real error here would be ... ?
> > That makes me wonder why this has been registered as the source of the freq
> > scale in the first place if we are to hit some serious issue. Would you be able
> > to give an example of any?
> If it gets here, that would be -ENODEV or -EIO from cppc_get_perf_ctrs(),
> which could possibly come from data corruption (no CPC descriptor) or a PCC
> failure.
> 
> I can't easily fake an error here, but the above -EFAULT path could
> happen when it luckily passes the FIE init.
> 
The change seems reasonable. Though I am wondering if some other errors might be
rather transient as well ? Like -EIO ?
Note, I'm not an expert here.

---
BR
Beata
> Jie
> > 
> > ---
> > BR
> > Beata
> >>  
> >> -- 
> >> 2.33.0
> >>
> >>
> >