Re: [PATCH v3 1/8] irqchip/mips-gic: Only register IPI domain when SMP is enabled

[Marc Zyngier <maz@kernel.org>]

On Tue, 05 Jul 2022 14:52:43 +0100,
Serge Semin <fancer.lancer@gmail.com> wrote:
> 
> Hi Samuel
> 
> On Fri, Jul 01, 2022 at 03:00:49PM -0500, Samuel Holland wrote:
> > The MIPS GIC irqchip driver may be selected in a uniprocessor
> > configuration, but it unconditionally registers an IPI domain.
> > 
> > Limit the part of the driver dealing with IPIs to only be compiled when
> > GENERIC_IRQ_IPI is enabled, which corresponds to an SMP configuration.
> 
> Thanks for the patch. Some comment is below.
> 
> > 
> > Reported-by: kernel test robot <lkp@intel.com>
> > Signed-off-by: Samuel Holland <samuel@sholland.org>
> > ---
> > 
> > Changes in v3:
> >  - New patch to fix build errors in uniprocessor MIPS configs
> > 
> >  drivers/irqchip/Kconfig        |  3 +-
> >  drivers/irqchip/irq-mips-gic.c | 80 +++++++++++++++++++++++-----------
> >  2 files changed, 56 insertions(+), 27 deletions(-)
> > 
> > diff --git a/drivers/irqchip/Kconfig b/drivers/irqchip/Kconfig
> > index 1f23a6be7d88..d26a4ff7c99f 100644
> > --- a/drivers/irqchip/Kconfig
> > +++ b/drivers/irqchip/Kconfig
> > @@ -322,7 +322,8 @@ config KEYSTONE_IRQ
> >  
> >  config MIPS_GIC
> >  	bool
> > -	select GENERIC_IRQ_IPI
> > +	select GENERIC_IRQ_IPI if SMP
> 
> > +	select IRQ_DOMAIN_HIERARCHY
> 
> It seems to me that the IRQ domains hierarchy is supposed to be
> created only if IPI is required. At least that's what the MIPS GIC
> driver implies. Thus we can go further and CONFIG_IRQ_DOMAIN_HIERARCHY
> ifdef-out the gic_irq_domain_alloc() and gic_irq_domain_free()
> methods definition together with the initialization:
> 
>  static const struct irq_domain_ops gic_irq_domain_ops = {
>  	.xlate = gic_irq_domain_xlate,
> +#ifdef CONFIG_IRQ_DOMAIN_HIERARCHY
>  	.alloc = gic_irq_domain_alloc,
>  	.free = gic_irq_domain_free,
> +#endif
>  	.map = gic_irq_domain_map,
> };
> 
> If the GENERIC_IRQ_IPI config is enabled, CONFIG_IRQ_DOMAIN_HIERARCHY
> will be automatically selected (see the config definition in
> kernel/irq/Kconfig). If the IRQs hierarchy is needed for some another
> functionality like GENERIC_MSI_IRQ_DOMAIN or GPIOs then they will
> explicitly enable the IRQ_DOMAIN_HIERARCHY config thus activating the
> denoted .alloc and .free methods definitions.
> 
> To sum up you can get rid of the IRQ_DOMAIN_HIERARCHY config
> force-select from this patch and make the MIPS GIC driver code a bit
> more coherent.
> 
> @Marc, please correct me if were wrong.

Either way probably works correctly, but Samuel's approach is more
readable IMO. It is far easier to reason about a high-level feature
(GENERIC_IRQ_IPI) than an implementation detail (IRQ_DOMAIN_HIERARCHY).

If you really want to save a handful of bytes, you can make the
callbacks conditional on GENERIC_IRQ_IPI, and be done with it. But
this can come as an additional patch.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.