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

[Jason Cooper]

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.

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.

thx,

Jason.