Re: [PATCH v2 4/6] genirq: Provide an IRQ affinity mask in non-SMP configs

[Samuel Holland]

On 6/18/22 4:01 AM, Marc Zyngier wrote:
> Hi Samuel,
> 
> On Thu, 16 Jun 2022 07:40:26 +0100,
> Samuel Holland <samuel@xxxxxxxxxxxx> wrote:
>>
>> IRQ affinity masks are not allocated in uniprocessor configurations.
>> This requires special case non-SMP code in drivers for irqchips which
>> have per-CPU enable or mask registers.
>>
>> Since IRQ affinity is always the same in a uniprocessor configuration,
>> we can still provide the correct affinity mask without allocating one
>> per IRQ. We can reuse the system-wide cpu_possible_mask.
>>
>> By returning a real cpumask from irq_data_get_affinity_mask even when
>> SMP is disabled, irqchip drivers which iterate over that mask will
>> automatically do the right thing.
>>
>> Signed-off-by: Samuel Holland <samuel@xxxxxxxxxxxx>
>> ---
>>
>> (no changes since v1)
>>
>>  include/linux/irq.h | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/include/linux/irq.h b/include/linux/irq.h
>> index 69ee4e2f36ce..d5e958b026aa 100644
>> --- a/include/linux/irq.h
>> +++ b/include/linux/irq.h
>> @@ -151,7 +151,9 @@ struct irq_common_data {
>>  #endif
>>  	void			*handler_data;
>>  	struct msi_desc		*msi_desc;
>> +#ifdef CONFIG_SMP
>>  	cpumask_var_t		affinity;
>> +#endif
>>  #ifdef CONFIG_GENERIC_IRQ_EFFECTIVE_AFF_MASK
>>  	cpumask_var_t		effective_affinity;
>>  #endif
>> @@ -881,7 +883,11 @@ static inline int irq_data_get_node(struct irq_data *d)
>>  
>>  static inline struct cpumask *irq_data_get_affinity_mask(struct irq_data *d)
>>  {
>> +#ifdef CONFIG_SMP
>>  	return d->common->affinity;
>> +#else
>> +	return &__cpu_possible_mask;
>> +#endif
> 
> I have a bad feeling about this one. Being in a !SMP configuration
> doesn't necessarily mean that __cpu_possible_mask only contains a
> single CPU, specially with things like CONFIG_INIT_ALL_POSSIBLE. I can
> also imagine an architecture populating this bitmap from firmware
> tables irrespective of the SMP status of the kernel.
> 
> Can't you use something like:
> 
> 	return cpumask_of(0);
> 
> which is guaranteed to be the right thing on !SMP configuration?

I can if I cast away the const. However I see a lot of:

    cpumask_copy(irq_data_get_affinity_mask(d), foo);

which I suppose is a great reason not to do what I am doing. The right solution
seems to be adding irq_data_update_affinity() to match
irq_data_update_effective_affinity(), and making both getters return a const
cpumask. Then I can use cpumask_of(0).

Regards,
Samuel