Re: [PATCH v2] PCI: rockchip: Avoid accessing PCIe registers with clocks gated

[Javier Martinez Canillas]

Hello Bjorn,

On 6/25/21 12:40 AM, Bjorn Helgaas wrote:
> [+cc Michal, Ley Foon, Jingoo, Thierry, Jonathan]
> 
> On Tue, Jun 08, 2021 at 10:04:09AM +0200, Javier Martinez Canillas wrote:
>> IRQ handlers that are registered for shared interrupts can be called at
>> any time after have been registered using the request_irq() function.
>>
>> It's up to drivers to ensure that's always safe for these to be called.
>>
>> Both the "pcie-sys" and "pcie-client" interrupts are shared, but since
>> their handlers are registered very early in the probe function, an error
>> later can lead to these handlers being executed before all the required
>> resources have been properly setup.
>>
>> For example, the rockchip_pcie_read() function used by these IRQ handlers
>> expects that some PCIe clocks will already be enabled, otherwise trying
>> to access the PCIe registers causes the read to hang and never return.
>>
>> The CONFIG_DEBUG_SHIRQ option tests if drivers are able to cope with their
>> shared interrupt handlers being called, by generating a spurious interrupt
>> just before a shared interrupt handler is unregistered.
>>
>> But this means that if the option is enabled, any error in the probe path
>> of this driver could lead to one of the IRQ handlers to be executed.
> 
> I'm not an IRQ expert, but I think this is an issue regardless of
> CONFIG_DEBUG_SHIRQ, isn't it?  Anything used by an IRQ handler should
> be initialized before the handler is registered.  CONFIG_DEBUG_SHIRQ
> is just a way to help find latent problems.
>

Yes, it's an issue regardless. It's just that this debug option tests if the
drivers aren't making the wrong assumption, exactly to find issues like this.

>> In a rockpro64 board, the following sequence of events happens:
>>
>>   1) "pcie-sys" IRQ is requested and its handler registered.
>>   2) "pcie-client" IRQ is requested and its handler registered.
>>   3) probe later fails due readl_poll_timeout() returning a timeout.
>>   4) the "pcie-sys" IRQ is unregistered.
>>   5) CONFIG_DEBUG_SHIRQ triggers a spurious interrupt.
>>   6) "pcie-client" IRQ handler is called for this spurious interrupt.
>>   7) IRQ handler tries to read PCIE_CLIENT_INT_STATUS with clocks gated.
>>   8) the machine hangs because rockchip_pcie_read() call never returns.
>>
>> To avoid cases like this, the handlers don't have to be registered until
>> very late in the probe function, once all the resources have been setup.
>>
>> So let's just move all the IRQ init before the pci_host_probe() call, that
>> will prevent issues like this and seems to be the correct thing to do too.
> 
> Previously we registered rockchip_pcie_subsys_irq_handler() and
> rockchip_pcie_client_irq_handler() before the PCIe clocks were
> enabled.  That's a problem because they depend on those clocks being
> enabled, and your patch fixes that.
>
> rockchip_pcie_legacy_int_handler() depends on rockchip->irq_domain,
> which isn't initialized until rockchip_pcie_init_irq_domain().
> Previously we registered rockchip_pcie_legacy_int_handler() as the
> handler for the "legacy" IRQ before rockchip_pcie_init_irq_domain().
> 
> I think you patch *also* fixes that problem, right?
>

Correct, that's why I moved the initialization and IRQ enable after that.
 
> I think this is also an issue with the following other drivers.  They all
> set the handler to something that uses an IRQ domain before they
> actually initialize the domain:

Yes, I agreed with your assessment and also noticed that others drivers have
similar issues. I just don't have any of those platforms to try to reproduce
the bugs and test a fix.

Best regards,
-- 
Javier Martinez Canillas
Software Engineer
New Platform Technologies Enablement team
RHEL Engineering