linux-kernel - Re: [PATCH] arm: document "mach-virt" platform.

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Message-ID: <52EA8853.8080709@arm.com>
Date:	Thu, 30 Jan 2014 17:13:55 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Ian Campbell <ian.campbell@...rix.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Kumar Gala <galak@...eaurora.org>,
	Olof Johansson <olof@...om.net>, Arnd Bergmann <arnd@...db.de>,
	Will Deacon <Will.Deacon@....com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm: document "mach-virt" platform.

Hi Ian,

On 30/01/14 16:11, Ian Campbell wrote:
> mach-virt has existed for a while but it is not written down what it actually
> consists of. Although it seems a bit unusual to document a binding for an
> entire platform since mach-virt is entirely virtual it is helpful to have
> something to refer to in the absence of a single concrete implementation.
> 
> I've done my best to capture the requirements based on the git log and my
> memory/understanding.
> 
> While here remove the xenvm dts example, the Xen tools will now build a
> suitable mach-virt compatible dts when launching the guest.
> 
> Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
> Cc: Rob Herring <robh+dt@...nel.org>
> Cc: Pawel Moll <pawel.moll@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Kumar Gala <galak@...eaurora.org>
> Cc: Olof Johansson <olof@...om.net>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Stefano Stabellini <stefano.stabellini@...citrix.com>
> Cc: devicetree@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-arm-kernel@...ts.infradead.org
> ---
> I'm not sure which tree this sort of thing should go though, sorry for the
> huge Cc.
> ---
>  .../devicetree/bindings/arm/mach-virt.txt          |   32 ++++++++
>  arch/arm/boot/dts/xenvm-4.2.dts                    |   81 --------------------
>  2 files changed, 32 insertions(+), 81 deletions(-)
>  create mode 100644 Documentation/devicetree/bindings/arm/mach-virt.txt
>  delete mode 100644 arch/arm/boot/dts/xenvm-4.2.dts
> 
> diff --git a/Documentation/devicetree/bindings/arm/mach-virt.txt b/Documentation/devicetree/bindings/arm/mach-virt.txt
> new file mode 100644
> index 0000000..562bcda
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/arm/mach-virt.txt
> @@ -0,0 +1,32 @@
> +* Mach-virt "Dummy Virtual Machine" platform
> +
> +"mach-virt" is the smallest, dumbest platform possible, to be used as
> +a guest for Xen, KVM and other hypervisors. It has no
> +properties/functionality of its own and is driven entirely by device
> +tree.
> +
> +This document defines the requirements for such a platform.
> +
> +* Required properties:
> +
> +- compatible: should be one of:
> +	"linux,dummy-virt"
> +	"xen,xenvm"
> +
> +In addition to the standard nodes (chosen, cpus, memory etc) the
> +platform is required to provide certain other basic functionality
> +which must be described in the device tree:
> +
> +    The platform must provide an ARM Generic Interrupt Controller
> +    (GIC), defined in Documentation/devicetree/bindings/arm/gic.txt.
> +
> +    The platform must provide ARM architected timer, defined in
> +    Documentation/devicetree/bindings/arm/arch_timer.txt.
> +
> +    If the platform is SMP then it must provide the Power State
> +    Coordination Interface (PSCI) described in
> +    Documentation/devicetree/bindings/arm/psci.txt.

I'm afraid I disagree with most of the above. The whole point of
mach-virt is to provide a shell for DT platforms. None of this hardware
is mandated. Instead, all the necessary information should be described
in DT.

Actually, mach-virt doesn't really stand for Virtual Machine. It stands
for virtual mach-* directory! Eventually, mach-virt should become the
default platform, the one we use when we don't match anything else in
the kernel

What you've described here are requirements for a hypervisor like Xen or
KVM. mach-virt itself shouldn't have any of that.

Cheers,

	M.
-- 
Jazz is not dead. It just smells funny...
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