linux-kernel - Re: [PATCH] cpufreq: scpi: compare against frequency instead of rate

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Message-ID: <8ebf8f26-c3d9-43c0-b417-ce3131a84eb4@stanley.mountain>
Date: Thu, 23 Jan 2025 16:04:13 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Sudeep Holla <sudeep.holla@....com>
Cc: zuoqian <zuoqian113@...il.com>,
	Ionela Voinescu <ionela.voinescu@....com>, rafael@...nel.org,
	viresh.kumar@...aro.org, cristian.marussi@....com,
	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: scpi: compare against frequency instead of rate

On Thu, Jan 23, 2025 at 12:16:50PM +0000, Sudeep Holla wrote:
> (for some reason I don't have the original email)
> 
> On Thu, Jan 23, 2025 at 02:12:14PM +0300, Dan Carpenter wrote:
> > On Thu, Jan 23, 2025 at 07:53:20AM +0000, zuoqian wrote:
> > > The CPU rate from clk_get_rate() may not be divisible by 1000
> > > (e.g., 133333333). But the rate calculated from frequency is always
> > > divisible by 1000 (e.g., 133333000).
> > > Comparing the rate causes a warning during CPU scaling:
> > > "cpufreq: __target_index: Failed to change cpu frequency: -5".
> > > When we choose to compare frequency here, the issue does not occur.
> > > 
> > > Signed-off-by: zuoqian <zuoqian113@...il.com>
> > > ---
> > >  drivers/cpufreq/scpi-cpufreq.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/cpufreq/scpi-cpufreq.c b/drivers/cpufreq/scpi-cpufreq.c
> > > index cd89c1b9832c..3bff4bb5ab4a 100644
> > > --- a/drivers/cpufreq/scpi-cpufreq.c
> > > +++ b/drivers/cpufreq/scpi-cpufreq.c
> > > @@ -39,8 +39,9 @@ static unsigned int scpi_cpufreq_get_rate(unsigned int cpu)
> > >  static int
> > >  scpi_cpufreq_set_target(struct cpufreq_policy *policy, unsigned int index)
> > >  {
> > > -	u64 rate = policy->freq_table[index].frequency * 1000;
> > 
> > policy->freq_table[index].frequency is a u32 so in this original
> > calculation, even though "rate" is declared as a u64, it can't actually
> > be more than UINT_MAX.
> >
> 
> Agreed and understood.
> 
> > > +	unsigned long freq = policy->freq_table[index].frequency;
> > >  	struct scpi_data *priv = policy->driver_data;
> > > +	u64 rate = freq * 1000;
> >
> > So you've fixed this by casting policy->freq_table[index].frequency
> > to unsigned long, which fixes the problem on 64bit systems but it still
> > remains on 32bit systems.  It would be better to declare freq as a u64.
> >
> 
> Just trying to understand if that matters. freq is in kHz as copied
> from policy->freq_table[index].frequency and we compare it with
> kHZ below as the obtained clock rate is divided by 1000. What am I
> missing ? If it helps, it can be renamed as freq_in_khz and even keep
> it as "unsigned int" as in struct cpufreq_frequency_table.
> 


I misunderstood the integer overflow bug because I read too much into the
fact that "rate" was declared as a u64.  It would have been fine to
declare it as a unsigned long.  The cpufreq internals don't support
anything more than ULONG_MAX.  I have heard someone say that new systems
are bumping up against the 4GHz limit but presumably that would only be
high end 64bit systems, not old 32bit system.

The ->freq_table[] frequency is in kHz so a u32 is fine.  I guess if we
get frequencies of a THz then we'll have to update that.  But when we
convert to Hz then we need a cast to avoid an integer overflow for systems
which are over the 4GHz boundary.

	unsigned long rate = (unsigned long)khz * 1000;

The second bug is that we need to compare kHz instead of Hz and that's
straight forward.

regards,
dan carpenter