linux-kernel - Re: [RFC PATCH 3/9] irqchip: GIC: Convert to EOImode == 1

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Date:	Wed, 25 Jun 2014 07:50:12 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <will.deacon@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	eric.auger@...aro.org
Subject: Re: [RFC PATCH 3/9] irqchip: GIC: Convert to EOImode == 1

On Wed, Jun 25, 2014 at 4:28 AM, Marc Zyngier <marc.zyngier@....com> wrote:
> So far, GICv2 has been used in with EOImode == 0. The effect of this
> mode is to perform the priority drop and the deactivation of the
> interrupt at the same time.
>
> While this works perfectly for Linux (we only have a single priority),
> it causes issues when an interrupt is forwarded to a guest, and when
> we want the guest to perform the EOI itself.
>
> For this case, the GIC architecture provides EOImode == 1, where:
> - A write to the EOI register drops the priority of the interrupt and leaves
> it active. Other interrupts at the same priority level can now be taken,
> but the active interrupt cannot be taken again
> - A write to the DIR marks the interrupt as inactive, meaning it can
> now be taken again.
>
> We only enable this feature when booted in HYP mode. Also, as most device
> trees are broken (they report the CPU interface size to be 4kB, while
> the GICv2 CPU interface size is 8kB), output a warning if we're booted
> in HYP mode, and disable the feature.

Why not fix-up the size so the feature can be enabled?

Rob
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