linux-kernel - Re: [PATCH v2 1/4] genirq/irqdomain: Allow partial trimming of irq

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Date:   Wed, 07 Oct 2020 09:53:49 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Sowjanya Komatineni <skomatineni@...dia.com>,
        Venkat Reddy Talla <vreddytalla@...dia.com>,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 1/4] genirq/irqdomain: Allow partial trimming of
 irq_data hierarchy

On 2020-10-07 09:05, Marc Zyngier wrote:
> On 2020-10-06 21:39, Thomas Gleixner wrote:
>> On Tue, Oct 06 2020 at 11:11, Marc Zyngier wrote:
>>> It appears that some HW is ugly enough that not all the interrupts
>>> connected to a particular interrupt controller end up with the same
>>> hierarchy repth (some of them are terminated early). This leaves
>> 
>>   depth?
>> 
>>> the irqchip hacker with only two choices, both equally bad:
>>> 
>>> - create discrete domain chains, one for each "hierarchy depth",
>>>   which is very hard to maintain
>>> 
>>> - create fake hierarchy levels for the shallow paths, leading
>>>   to all kind of problems (what are the safe hwirq values for these
>>>   fake levels?)
>>> 
>>> Instead, let's offer the possibility to cut short a single interrupt
>> 
>> s/let's offer/implement/
> 
> Thanks for that, I'll fix it locally.
> 
> [...]
> 
>> 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.
> 
>> 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.

          M.
-- 
Jazz is not dead. It just smells funny...