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

[Geert Uytterhoeven]

On Thu, Mar 17, 2016 at 5:20 PM, Jon Hunter <jonathanh@xxxxxxxxxx> wrote:
> On 17/03/16 15:18, Jason Cooper wrote:
>> On Thu, Mar 17, 2016 at 03:04:01PM +0000, Jon Hunter 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.

Indeed (also for sh73a0 and r8a7779).

>> This sounds like a textbook case for adding a boolean dt property.  If
>> "can-set-ppi-type" is absent (old DT blobs and new blobs without the
>> ability), warn and return zero.  If it's present, the driver can set the
>> type, returning errors as encountered.
>
> True. However, if we did have this "can-set-ppi-type" property set for a
> device, it really should never fail (unless someone specified it
> incorrectly). So I am trying to understand the value in adding a new DT
> property.

Do we really want to add properties that basically indicate that a description
in DT is correct?

Alternatively, it can be fixed in the kernel in a DT quirk (if SoC == xxx then
fix TWD).

> Please note that gic_configure_irq() never used to return an error and
> only when adding support for setting the type of PPIs was this added.
> However, given that this has gone unnoticed and does not have a real
> functional impact on the device behaviour, I wonder now if this function
> should return an error? Yes, ideally, it should, but does it still make
> sense?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds