CONFIG_DEBUG_SHIRQ and PM

[Ezequiel Garcia]

On 26 August 2015 at 17:03, Felipe Balbi <balbi at ti.com> wrote:
> On Wed, Aug 26, 2015 at 04:53:27PM -0300, Ezequiel Garcia wrote:
>> On 26 August 2015 at 16:38, Felipe Balbi <balbi at ti.com> wrote:
>> > Hi,
>> >
>> > On Wed, Aug 26, 2015 at 04:29:52PM -0300, Ezequiel Garcia wrote:
>> >> Felipe,
>> >>
>> >> On 25 August 2015 at 16:58, Felipe Balbi <balbi at ti.com> wrote:
>> >> > Hi Ingo,
>> >> >
>> >> > I'm facing an issue with CONFIG_DEBUG_SHIRQ and pm_runtime when using
>> >> > devm_request_*irq().
>> >> >
>> >>
>> >> I may be jumping on the gun here, but I believe here's your problem.
>> >> Using devm_request_irq with shared IRQs is not a good idea.
>> >>
>> >> Or at least, it forces you to handle interrupts after your device
>> >> is _completely_ removed (e.g. your IRQ cookie could be bogus).
>> >>
>> >> AFAICS, the CONFIG_DEBUG_SHIRQ option is just triggering a couple
>> >> spurious interrupts, that are expected to happen anyway.
>> >>
>> >> Your IRQ is shared, and so you may get any IRQ at any time,
>> >> coming from another device (not yours).
>> >>
>> >> So, if I'm right, my suggestion is simple: use request_irq/free_irq
>> >> and free your IRQ before you disable your clocks, remove your device,
>> >> etc.
>> >
>> > yeah, that's just a workaround though. Specially with IRQ flags coming
>> > from DT, driver might have no knowledge that its IRQ is shared to start
>> > with.
>> >
>>
>> Really? Is there any way the DT can set the IRQF_SHARED flag?
>> The caller of devm_request_irq / request_irq needs to pass the irqflags,
>> so I'd say the driver is perfectly aware of the IRQ being shared or not.
>>
>> Maybe you can clarify what I'm missing here.
>
> hmm, that's true. Now that I look at it, DT can pass triggering flags.
>
>> > Besides, removing devm_* is just a workaround to the real problem. It
>> > seems, to me at least, that drivers shouldn't be calling
>> > pm_runtime_put_sync(); pm_runtime_disable() from their ->remove(),
>> > rather the bus driver should be responsible for doing so; much
>> > usb_bus_type and pci_bus_type do. Of course, this has the side effect of
>> > requiring buses to make sure that by the time ->probe() is called, that
>> > device's clocks are stable and running and the device is active.
>> >
>> > However, that's not done today for the platform_bus_type and, frankly,
>> > that would be somewhat of a complex problem to solve, considering that
>> > different SoCs integrate IPs the way they want.
>> >
>> > Personally, I think removing devm_* is but a workaround to the real
>> > thing we're dealing with.
>> >
>>
>> I don't see any problems here: if your interrupt is shared, then you must
>> be prepared to handle it any time, coming from any sources (not only
>> your device). And CONFIG_DEBUG_SHIRQ does exactly that, in order to
>> make sure all the drivers passing IRQF_SHARED comply with that rule.
>
> you need to be sure of that with non-shared IRQs anyway.

Not entirely. If your IRQ is not shared, then you usually have a register
to enable or unmask your peripheral interrupts. So the driver is in control
of when it will get interrupts.

If the IRQ is shared, this won't do. This is what I mean by "shared IRQs
must be prepared to receive an interrupt any time", in the sense that
the driver has no way of preventing IRQs (because they may be
coming from any source).

In the same sense, shared IRQs handlers need to double-check
the IRQ is coming to the current device by checking some IRQ
status register to see if there's pending work.

> Also, an IRQ
> which isn't shared in SoC A, might become shared in SoC B which uses the
> same IP.
>
>> So you either avoid using devm_request_irq, or you prepare your handler
>> accordingly to be ready to handle an interrupt _any time_.
>
> the handler is ready to handle at any time, what isn't correct is the
> fact that clocks get gated before IRQ is freed.
>
> There should be no such special case as "if your handler is shared,
> don't use devm_request_*irq()" because if we just disable PM_RUNTIME, it
> works as expected anyway.
>

Yeah, I meant to say: if you use devm_request_irq with IRQF_SHARED
then you _must_ be prepared to get an IRQ *after* your remove() has
been called.

Let's consider this snippet from tw68:

static irqreturn_t tw68_irq(int irq, void *dev_id)
{
        struct tw68_dev *dev = dev_id;
        u32 status, orig;
        int loop;

        status = orig = tw_readl(TW68_INTSTAT) & dev->pci_irqmask;
        [etc]
}

The IRQ handler accesses the device struct and then
reads through PCI. So if you use devm_request_irq
you need to make sure the device struct is still allocated
after remove(), and the PCI read won't stall or crash.

Interestingly, tw68 uses devm_request_irq with IRQF_SHARED :-)

Still, I don't think that's a good idea, since it relies on
the IRQ being freed *before* the device struct.
-- 
Ezequiel García, VanguardiaSur
www.vanguardiasur.com.ar