Re: [PATCH] cpufreq: Fix implicit type conversion
From: "Rafael J. Wysocki" <rafael@kernel.org>
Date: 2021-10-29 16:07:34
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On Fri, Oct 29, 2021 at 5:51 AM Jiasheng Jiang [off-list ref] 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.
quoted hunk ↗ jump to hunk
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 <redacted> --- 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