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

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Date:   Thu, 21 Sep 2023 12:11:39 +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 01:23:30PM +0200, Lorenzo Pieralisi wrote:
> 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.

Quick update, bindings filed, I will code against it but we should
not merge anything till it is approved (could be missing v6.7 timeline).

https://bugzilla.tianocore.org/show_bug.cgi?id=4557

Lorenzo