linux-kernel - Re: [PATCH] cpufreq: scmi: Skip SCMI devices that aren't used by the CPUs

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Message-ID: <Z/6NB2QRvRrqwgcQ@hu-mdtipton-lv.qualcomm.com>
Date: Tue, 15 Apr 2025 09:44:55 -0700
From: Mike Tipton <quic_mdtipton@...cinc.com>
To: Peng Fan <peng.fan@....nxp.com>
CC: Sudeep Holla <sudeep.holla@....com>,
        Cristian Marussi
	<cristian.marussi@....com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Viresh
 Kumar <viresh.kumar@...aro.org>, <arm-scmi@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <linux-pm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: scmi: Skip SCMI devices that aren't used by the
 CPUs

On Tue, Apr 15, 2025 at 05:06:55PM +0800, Peng Fan wrote:
> On Fri, Apr 11, 2025 at 02:29:41PM -0700, Mike Tipton wrote:
> >Currently, all SCMI devices with performance domains attempt to register
> >a cpufreq driver, even if their performance domains aren't used to
> >control the CPUs. The cpufreq framework only supports registering a
> >single driver, so only the first device will succeed. And if that device
> >isn't used for the CPUs, then cpufreq will scale the wrong domains.
> >
> >To avoid this, return early from scmi_cpufreq_probe() if the probing
> >SCMI device isn't referenced by the CPU device phandles.
> >
> >This keeps the existing assumption that all CPUs are controlled by a
> >single SCMI device.
> >
> >Signed-off-by: Mike Tipton <quic_mdtipton@...cinc.com>
> >---
> > drivers/cpufreq/scmi-cpufreq.c | 29 +++++++++++++++++++++++++++++
> > 1 file changed, 29 insertions(+)
> >
> >diff --git a/drivers/cpufreq/scmi-cpufreq.c b/drivers/cpufreq/scmi-cpufreq.c
> >index 944e899eb1be..7981a879974b 100644
> >--- a/drivers/cpufreq/scmi-cpufreq.c
> >+++ b/drivers/cpufreq/scmi-cpufreq.c
> >@@ -393,6 +393,32 @@ static struct cpufreq_driver scmi_cpufreq_driver = {
> > 	.set_boost	= cpufreq_boost_set_sw,
> > };
> > 
> >+static bool scmi_dev_used_by_cpus(struct device *scmi_dev)
> >+{
> >+	struct device_node *scmi_np = scmi_dev->of_node;
> >+	struct device_node *np;
> >+	struct device *cpu_dev;
> >+	int cpu, idx;
> >+
> >+	for_each_possible_cpu(cpu) {
> >+		cpu_dev = get_cpu_device(cpu);
> >+		if (!cpu_dev)
> >+			continue;
> >+
> >+		np = cpu_dev->of_node;
> >+
> >+		if (of_parse_phandle(np, "clocks", 0) == scmi_np)
> >+			return true;
> >+
> >+		idx = of_property_match_string(np, "power-domain-names", "perf");
> >+
> >+		if (of_parse_phandle(np, "power-domains", idx) == scmi_np)
> >+			return true;
> >+	}
> >+
> >+	return false;
> >+}
> >+
> > static int scmi_cpufreq_probe(struct scmi_device *sdev)
> > {
> > 	int ret;
> >@@ -404,6 +430,9 @@ static int scmi_cpufreq_probe(struct scmi_device *sdev)
> > 	if (!handle)
> > 		return -ENODEV;
> > 
> >+	if (!scmi_dev_used_by_cpus(dev))
> >+		return 0;
> 
> Should 'return -ENOTSUPP' be used here?
> There is no need to mark the probe success.

Returning -ENOTSUPP will add noise in the logs from probe failures, for
example:

    scmi-cpufreq scmi_dev.4: probe with driver scmi-cpufreq failed with error -524

These are "expected" failures, so this would be misleading. However, we
could return -ENODEV instead which doesn't log anything by default. It
uses a dev_dbg() in that case:

    scmi-cpufreq scmi_dev.4: probe with driver scmi-cpufreq rejects match -19

Returning -ENODEV seems more appropriate. I can make that change.