Re: [PATCH v2 1/4] genirq/irqdomain: Allow partial trimming of irq_data hierarchy

[Thomas Gleixner]

On Wed, Oct 07 2020 at 09:53, Marc Zyngier wrote:
> On 2020-10-07 09:05, Marc Zyngier wrote:
>> On 2020-10-06 21:39, Thomas Gleixner wrote:
>>> This is butt ugly, really. Especially the use case where the tegra PMC
>>> domain removes itself from the hierarchy from .alloc()
>> 
>> I don't disagree at all. It is both horrible and dangerous.
>> 
>> My preference would have been to split the PMC domain into discrete
>> domains, each one having having its own depth. But that's incredibly
>> hard to express in DT, and would break the combination of old/new
>> DT and kernel.

Moo.

>>> That said, I don't have a better idea either. Sigh...
>> 
>> A (very minor) improvement would be to turn the trim call in the PMC
>> driver into a flag set in the first invalid irq_data structure, and
>> let __irq_domain_alloc_irqs() do the dirty work.
>> 
>> Still crap, but at least would prevent some form of abuse. Thoughts?
>
> Actually, I wonder whether we can have a more general approach:
>
> A partial hierarchy that doesn't have an irq_data->chip pointer
> populated cannot be valid. So I wonder if the least ugly thing to do
> is to just drop any messing about in the PMC driver, and instead to
> let __irq_domain_alloc_irqs() do the culling, always, by looking for a
> NULL pointer in irq_data->chip.
>
> Not any less ugly, but at least doesn't need any driver intervention.

I like that approach.

Thanks,

        tglx