linux-kernel - Re: [PATCH v2 3/4] cpufreq: mediatek: add Mediatek cpufreq driver

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Date:	Tue, 10 Mar 2015 09:57:07 +0800
From:	Pi-Cheng Chen <pi-cheng.chen@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Mike Turquette <mturquette@...aro.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Will Deacon <will.deacon@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Chen Fan <fan.chen@...iatek.com>,
	Howard Chen <ibanezchen@...il.com>,
	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	linux-mediatek@...ts.infradead.org,
	Kumar Gala <galak@...eaurora.org>,
	"Joe.C" <yingjoe.chen@...iatek.com>,
	Eddie Huang <eddie.huang@...iatek.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 3/4] cpufreq: mediatek: add Mediatek cpufreq driver

On 10 March 2015 at 00:28, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Wed, Mar 04, 2015 at 04:49:15PM +0800, pi-cheng.chen wrote:
>> +static int cpu_opp_table_get_freq_index(unsigned int freq)
>> +{
>> +     struct cpu_opp_table *opp_tbl = dvfs_info->opp_tbl;
>> +     int i;
>> +
>> +     for (i = 0; opp_tbl[i].freq != 0; i++) {
>> +             if (opp_tbl[i].freq >= freq)
>> +                     return i;
>> +     }
>> +
>> +     return -1;
>
> My "return -1" detector fired on this...
>
>> +static int cpu_opp_table_get_volt_index(unsigned int volt)
>> +{
>> +     struct cpu_opp_table *opp_tbl = dvfs_info->opp_tbl;
>> +     int i;
>> +
>> +     for (i = 0; opp_tbl[i].vproc != -1; i++)
>> +             if (opp_tbl[i].vproc >= volt)
>> +                     return i;
>> +
>> +     return -1;
>
> And this.
>
>> +static int mtk_cpufreq_notify(struct notifier_block *nb,
>> +                           unsigned long action, void *data)
>> +{
>> +     struct cpufreq_freqs *freqs = data;
>> +     struct cpu_opp_table *opp_tbl = dvfs_info->opp_tbl;
>> +     int old_vproc, new_vproc, old_index, new_index;
>> +
>> +     if (!cpumask_test_cpu(freqs->cpu, &dvfs_info->cpus))
>> +             return NOTIFY_DONE;
>> +
>> +     old_vproc = regulator_get_voltage(dvfs_info->proc_reg);
>> +     old_index = cpu_opp_table_get_volt_index(old_vproc);
>> +     new_index = cpu_opp_table_get_freq_index(freqs->new * 1000);
>> +     new_vproc = opp_tbl[new_index].vproc;
>
> Let's say that cpu_opp_table_get_freq_index() returns -1.  We then
> do no error checking on this, and access the memory immediately
> preceding opp_tbl[0].
>
> Since we'll be loading garbage from opp_tbl[-1], this probably means
> that mtk_cpufreq_voltage_trace() will go wrong.  Your method of using
> the vproc values to work out which direction we should walk between
> old_index...new_index means that we could end up walking through
> almost the whole UINT_MAX range to wrap around to the new index.
>
> Yet again, "return -1" proves to be a sure sign of a bug.

Hi Russell,

Thanks for your reviewing. I'll fix it in next version.

Best Regards,
Pi-Cheng

>
> --
> FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
> according to speedtest.net.
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