linux-kernel - Re: [PATCH v2 0/2] irqchip/gic-v3: Enable non-coherent GIC designs probing

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Message-ID: <ZPhhMvyx8lChHE0v@lpieralisi>
Date:   Wed, 6 Sep 2023 13:23:30 +0200
From:   Lorenzo Pieralisi <lpieralisi@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        devicetree@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Robin Murphy <robin.murphy@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Fang Xiang <fangxiang3@...omi.com>
Subject: Re: [PATCH v2 0/2] irqchip/gic-v3: Enable non-coherent GIC designs
 probing

On Wed, Sep 06, 2023 at 10:52:01AM +0100, Marc Zyngier wrote:
> On 2023-09-06 10:41, Lorenzo Pieralisi wrote:
> > This series is v2 of a previous version[1].
> > 
> > v1 -> v2:
> > 	- Updated DT bindings as per feedback
> > 	- Updated patch[2] to use GIC quirks infrastructure
> > 
> > [1]
> > https://lore.kernel.org/all/20230905104721.52199-1-lpieralisi@kernel.org
> > 
> > Original cover letter
> > ---
> > The GICv3 architecture specifications provide a means for the
> > system programmer to set the shareability and cacheability
> > attributes the GIC components (redistributors and ITSes) use
> > to drive memory transactions.
> > 
> > Albeit the architecture give control over shareability/cacheability
> > memory transactions attributes (and barriers), it is allowed to
> > connect the GIC interconnect ports to non-coherent memory ports
> > on the interconnect, basically tying off shareability/cacheability
> > "wires" and de-facto making the redistributors and ITSes non-coherent
> > memory observers.
> > 
> > This series aims at starting a discussion over a possible solution
> > to this problem, by adding to the GIC device tree bindings the
> > standard dma-noncoherent property. The GIC driver uses the property
> > to force the redistributors and ITSes shareability attributes to
> > non-shareable, which consequently forces the driver to use CMOs
> > on GIC memory tables.
> > 
> > On ARM DT DMA is default non-coherent, so the GIC driver can't rely
> > on the generic DT dma-coherent/non-coherent property management layer
> > (of_dma_is_coherent()) which would default all GIC designs in the field
> > as non-coherent; it has to rely on ad-hoc dma-noncoherent property
> > handling.
> > 
> > When a consistent approach is agreed upon for DT an equivalent binding
> > will
> > be put forward for ACPI based systems.
> 
> What is the plan for this last point? I'd like to see at least
> a proposal before taking this series in.

Absolutely, I am starting a thread on related MADT changes, should not take
too long.

Lorenzo