Re: [PATCH 04/15] irqchip/gic: WARN if setting the interrupt type fails

[Jon Hunter <jonathanh@nvidia.com>]

Hi Mark,

On 09/04/16 11:58, Marc Zyngier wrote:
> On Thu, 17 Mar 2016 15:04:01 +0000
> Jon Hunter <jonathanh@nvidia.com> wrote:
> 
>>
>> On 17/03/16 14:51, Thomas Gleixner wrote:
>>> On Thu, 17 Mar 2016, Jon Hunter wrote:
>>>
>>>> Setting the interrupt type for private peripheral interrupts (PPIs) may
>>>> not be supported by a given GIC because it is IMPLEMENTATION DEFINED
>>>> whether this is allowed. There is no way to know if setting the type is
>>>> supported for a given GIC and so the value written is read back to
>>>> verify it matches the desired configuration. If it does not match then
>>>> an error is return.
>>>>
>>>> There are cases where the interrupt configuration read from firmware
>>>> (such as a device-tree blob), has been incorrect and hence
>>>> gic_configure_irq() has returned an error. This error has gone
>>>> undetected because the error code returned was ignored but the interrupt
>>>> still worked fine because the configuration for the interrupt could not
>>>> be overwritten.
>>>>
>>>> Given that this has done undetected and we should only fail to set the
>>>> type for PPIs whose configuration cannot be changed anyway, don't return
>>>> an error and simply WARN if this fails. This will allows us to fix up any
>>>> places in the kernel where we should be checking the return status and
>>>> maintain back compatibility with firmware images that may have incorrect
>>>> interrupt configurations.
>>>
>>> Though silently returning 0 is really the wrong thing to do. You can add the
>>> warn, but why do you want to return success?
>>
>> Yes that would be the correct thing to do I agree. However, the problem
>> is that if we do this, then after the patch "irqdomain: Don't set type
>> when mapping an IRQ" is applied, we may break interrupts for some
>> existing device-tree binaries that have bad configuration (such as omap4
>> and tegra20/30 ... see patches 1 and 2) that have gone unnoticed. So it
>> is a back compatibility issue.
>>
>> If you are wondering why these interrupts break after "irqdomain: Don't
>> set type when mapping an IRQ", it is because today
>> irq_create_fwspec_mapping() does not check the return code from setting
>> the type, but if we defer setting the type until __setup_irq() which
>> does check the return code, then all of a sudden interrupts that were
>> working (even with bad configurations) start to fail.
>>
>> The reason why I opted not to return an error code from
>> gic_configure_irq() is it really can't fail. The failure being reported
>> does not prevent the interrupt from working, but tells you your
>> configuration does not match the hardware setting which you cannot
>> overwrite.
>>
>> So to maintain back compatibility and avoid any silent errors, I opted
>> to make it a WARN and not return an error.
>>
>> If people are ok with potentially breaking interrupts for device-tree
>> binaries with bad settings, then I am ok to return an error here.
> 
> I think we need to phase things. Let's start with warning people for a
> few kernel releases. Actively maintained platforms will quickly address
> the issue (fixing their DT). As I see it, this issue seems rather
> widespread (even kvmtool outputs a DT with the wrong triggering
> information).
> 
> Once we've fixed the bulk of the platforms and virtual environments, we
> can start thinking about making it fail harder.

Ok, so are you OK with this patch as-is? If so, can I add your ACK?

Cheers
Jon