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

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Date:	Thu, 17 Mar 2016 15:04:01 +0000
From:	Jon Hunter <jonathanh@...dia.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <marc.zyngier@....com>,
	Benoît Cousson <bcousson@...libre.com>,
	Tony Lindgren <tony@...mide.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"Stephen Warren" <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Kevin Hilman <khilman@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Linus Walleij <linus.walleij@...aro.org>,
	<linux-tegra@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/15] irqchip/gic: WARN if setting the interrupt type
 fails

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.

Cheers
Jon