On 17/03/16 15:18, Jason Cooper wrote:

On Thu, Mar 17, 2016 at 03:04:01PM +0000, Jon Hunter wrote:

quoted

On 17/03/16 14:51, Thomas Gleixner wrote:

quoted

On Thu, 17 Mar 2016, Jon Hunter wrote:

quoted

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.

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.

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?

Cheers
Jon