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

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Message-ID: <02226efd42fdb249b81c01eb702100e9@kernel.org>
Date:   Wed, 07 Oct 2020 13:23:58 +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 09:53, 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.
>> 
>>> 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.

[still talking to myself...]

I implemented that, and it has the advantage of placing the hack in a
single location. It even booted on a garden variety of systems.

I'll post an updated series, and we can compare the various levels
of ugliness.

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