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SubjectRe: [PATCH] cpufreq: Fix implicit type conversion
On Fri, Oct 29, 2021 at 5:51 AM Jiasheng Jiang <jiasheng@iscas.ac.cn> wrote:
>
> The variable 'cpu' and 'j' are defined as unsigned int.
> However in the for_each_cpu, their values are assigned to -1.
> That doesn't make sense

Yes, it does.

The binary representation of -1 is an all-ones value of the size of
int. It is perfectly valid to store that value in an unsigned int
variable.

> and in the cpumask_next() they are implicitly type conversed to int.

However, the return type of cpumask_next() is unsigned int.

> It is universally accepted that the implicit type conversion is terrible.

I wouldn't say "terrible", but yes, it is risky when dealing with
variables of different sizes and possible sign-extensions.

In this particular case, I don't see a problem.

> Also, having the good programming custom will set an example for
> others.
> Thus, it might be better to change the definition of 'cpu' and 'j'
> from unsigned int to int.
>
> Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
> ---
> kernel/sched/cpufreq_schedutil.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 4f09afd..4aff4b6 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -409,7 +409,7 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
> struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> struct cpufreq_policy *policy = sg_policy->policy;
> unsigned long util = 0, max = 1;
> - unsigned int j;
> + int j;
>
> for_each_cpu(j, policy->cpus) {
> struct sugov_cpu *j_sg_cpu = &per_cpu(sugov_cpu, j);
> @@ -746,7 +746,7 @@ static int sugov_start(struct cpufreq_policy *policy)
> {
> struct sugov_policy *sg_policy = policy->governor_data;
> void (*uu)(struct update_util_data *data, u64 time, unsigned int flags);
> - unsigned int cpu;
> + int cpu;
>
> sg_policy->freq_update_delay_ns = sg_policy->tunables->rate_limit_us * NSEC_PER_USEC;
> sg_policy->last_freq_update_time = 0;
> @@ -783,7 +783,7 @@ static int sugov_start(struct cpufreq_policy *policy)
> static void sugov_stop(struct cpufreq_policy *policy)
> {
> struct sugov_policy *sg_policy = policy->governor_data;
> - unsigned int cpu;
> + int cpu;
>
> for_each_cpu(cpu, policy->cpus)
> cpufreq_remove_update_util_hook(cpu);
> --
> 2.7.4
>

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