linux-kernel - Re: [PATCH 4/4] base/drivers/topology: Default dmpis-mhz if they are not set in DT

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Date:   Tue, 30 Oct 2018 09:39:56 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     rjw@...ysocki.net, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org,
        Chris Redpath <chris.redpath@...aro.org>,
        Quentin Perret <quentin.perret@...aro.org>,
        Amit Kucheria <amit.kucheria@...aro.org>,
        Nicolas Dechesne <nicolas.dechesne@...aro.org>,
        Niklas Cassel <niklas.cassel@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH 4/4] base/drivers/topology: Default dmpis-mhz if they are
 not set in DT

On 30/10/2018 08:13, Viresh Kumar wrote:
> On 29-10-18, 17:23, Daniel Lezcano wrote:
>> In the case of assymetric SoC with the same micro-architecture, we
>> have a group of CPUs with smaller OPPs than the other group. One
>> example is the 96boards dragonboard 820c. There is no dmips/MHz
>> difference between both groups, so no need to specify the values in
>> the DT. Unfortunately, without these defined, there is no scaling
>> capacity comutation triggered, so we need to write
>> 'capacity-dmips-mhz' for each CPU with the same value in order to
>> force the scaled capacity computation.
>>
>> Fix this by setting a default capacity to SCHED_CAPACITY_SCALE, if no
>> 'capacity-dmips-mhz' is defined in the DT.
>>
>> This was tested on db820c:
>>  - specified values in the DT (correct results)
>>  - partial values defined in the DT (error + fallback to defaults)
>>  - no specified values in the DT (correct results)
>>
>> correct results are:
>>   cat /sys/devices/system/cpu/cpu*/cpu_capacity
>>    758
>>    758
>>   1024
>>   1024
>>
>>   ... respectively for CPU0, CPU1, CPU2 and CPU3.
>>
>> That reflects the capacity for the max frequencies 1593600 and 2150400.
>>
>> Cc: Chris Redpath <chris.redpath@...aro.org>
>> Cc: Quentin Perret <quentin.perret@...aro.org>
>> Cc: Viresh Kumar <viresh.kumar@...aro.org>
>> Cc: Amit Kucheria <amit.kucheria@...aro.org>
>> Cc: Nicolas Dechesne <nicolas.dechesne@...aro.org>
>> Cc: Niklas Cassel <niklas.cassel@...aro.org>
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
>> ---
>>  drivers/base/arch_topology.c | 27 ++++++++++++++++++++++++++-
>>  1 file changed, 26 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
>> index 7311641..7d594a6 100644
>> --- a/drivers/base/arch_topology.c
>> +++ b/drivers/base/arch_topology.c
>> @@ -205,6 +205,21 @@ static struct notifier_block init_cpu_capacity_notifier = {
>>  	.notifier_call = init_cpu_capacity_callback,
>>  };
>>  
>> +static int topology_set_default_capacity(void)
>> +{
>> +	int cpu;
>> +
>> +	raw_capacity = kzalloc(num_possible_cpus() * sizeof(*raw_capacity),
>> +			       GFP_KERNEL);
> 
> kmalloc ?
> 
>> +	if (!raw_capacity)
>> +		return -ENOMEM;
>> +
>> +	for_each_possible_cpu(cpu)
>> +		raw_capacity[cpu] = SCHED_CAPACITY_SCALE;
> 
> It makes it look as if it is important to set default values to
> SCHED_CAPACITY_SCALE while that is not the case. Maybe add a comment
> that all CPUs are assumed to have same capacity now. Maybe initialize
> to 1 instead ? Not sure if that would be better or not.

SCHED_CAPACITY_SCALE is the default value in this file.

eg.

DEFINE_PER_CPU(unsigned long, cpu_scale) = SCHED_CAPACITY_SCALE
...
pr_err("cpu_capacity: partial information: fallback to 1024 for all
CPUs\n");
...

So I prefer to use the SCHED_CAPACITY_SCALE for consistency.

>> +
>> +	return 0;
>> +}
>> +
>>  static int __init register_cpufreq_notifier(void)
>>  {
>>  	int ret;
>> @@ -214,9 +229,19 @@ static int __init register_cpufreq_notifier(void)
>>  	 * until we have the necessary code to parse the cpu capacity, so
>>  	 * skip registering cpufreq notifier.
>>  	 */
>> -	if (!acpi_disabled || !raw_capacity)
>> +	if (!acpi_disabled)
>>  		return -EINVAL;
>>  
>> +	if (!raw_capacity) {
>> +
>> +		pr_info("cpu_capacity: No capacity defined in DT, set default "
>> +		       "values to %ld\n", SCHED_CAPACITY_SCALE);
> 
> So we will end up doing this also for the case where the DT is
> partially filled with the capacity info and we already printed error
> messages for it. Should we really do that ?

Yes, it is the default behavior as stated in the documentation:

"capacity-dmips-mhz property is all-or-nothing: if it is specified for a
cpu node, it has to be specified for every other cpu nodes, or the
system will fall back to the default capacity value for every CPU."

and also in the error message:

pr_err("cpu_capacity: partial information: fallback to 1024 for all
CPUs\n");

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