linux-kernel - Re: Why set .suppress_bind_attrs even though .remove() implemented?

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Message-ID: <20220722171706.GA1911557@bhelgaas>
Date:   Fri, 22 Jul 2022 12:17:06 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Pali Rohár <pali@...nel.org>,
        Johan Hovold <johan+linaro@...nel.org>,
        Kishon Vijay Abraham I <kishon@...com>,
        Xiaowei Song <songxiaowei@...ilicon.com>,
        Binghui Wang <wangbinghui@...ilicon.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Ryder Lee <ryder.lee@...iatek.com>,
        Jianjun Wang <jianjun.wang@...iatek.com>,
        linux-pci@...r.kernel.org,
        Krzysztof Wilczyński <kw@...ux.com>,
        Ley Foon Tan <ley.foon.tan@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: Why set .suppress_bind_attrs even though .remove() implemented?

On Fri, Jul 22, 2022 at 06:06:07PM +0100, Marc Zyngier wrote:
> On Fri, 22 Jul 2022 15:39:05 +0100,
> Bjorn Helgaas <helgaas@...nel.org> wrote:
> > 
> > [+cc Marc, can you clarify when we need irq_dispose_mapping()?]
> 
> In general, interrupt controllers should not have to discard mappings
> themselves, just like they rarely create mappings themselves. That's
> usually a different layer that has created it (DT, for example).
> 
> The problem is that these mappings persist even if the interrupt has
> been released by the driver (it called free_irq()), and the IRQ number
> can be further reused. The client driver could dispose of the mapping
> after having released the IRQ, but nobody does that in practice.
> 
> From the point of view of the controller, there is no simple way to
> tell when an interrupt is "unused". And even if a driver was
> overzealous and called irq_dispose_mapping() on all the possible
> mappings (and made sure no mapping could be created in parallel), this
> could result in a bunch of dangling pointers should a client driver
> still have the interrupt requested.
> 
> Fixing this is pretty hard, as IRQ descriptors are leaky (you can
> either have a pointer to one, or just an IRQ number -- they are
> strictly equivalent). So in general, being able to remove an interrupt
> controller driver is at best fragile, and I'm trying not to get more
> of this in the tree.

Thank you!

How do we identify an interrupt controller driver?  Apparently some of
these PCIe controller drivers also include an interrupt controller
driver, but I don't know what to look for to find them.

Bjorn