linux-kernel - Re: [PATCH v4 14/14] Documentation: KHO: Add memblock bindings

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Message-ID: <Z6kTCvex3DGhB-3C@kernel.org>
Date: Sun, 9 Feb 2025 22:41:46 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: linux-kernel@...r.kernel.org, Alexander Graf <graf@...zon.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Anthony Yznaga <anthony.yznaga@...cle.com>,
	Arnd Bergmann <arnd@...db.de>, Ashish Kalra <ashish.kalra@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Borislav Petkov <bp@...en8.de>,
	Catalin Marinas <catalin.marinas@....com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Eric Biederman <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...hat.com>, James Gowans <jgowans@...zon.com>,
	Jonathan Corbet <corbet@....net>,
	Mark Rutland <mark.rutland@....com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Pratyush Yadav <ptyadav@...zon.de>,
	Rob Herring <robh+dt@...nel.org>, Rob Herring <robh@...nel.org>,
	Saravana Kannan <saravanak@...gle.com>,
	Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tom Lendacky <thomas.lendacky@....com>,
	Usama Arif <usama.arif@...edance.com>,
	Will Deacon <will@...nel.org>, devicetree@...r.kernel.org,
	kexec@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org,
	linux-doc@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org
Subject: Re: [PATCH v4 14/14] Documentation: KHO: Add memblock bindings

On Sun, Feb 09, 2025 at 04:23:09PM +0100, Krzysztof Kozlowski wrote:
> On 09/02/2025 16:10, Mike Rapoport wrote:
> > On Sun, Feb 09, 2025 at 11:29:41AM +0100, Krzysztof Kozlowski wrote:
> >> On 06/02/2025 14:27, Mike Rapoport wrote:
> >>> From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> >>>
> >>> We introduced KHO into Linux: A framework that allows Linux to pass
> >>> metadata and memory across kexec from Linux to Linux. KHO reuses fdt
> >>> as file format and shares a lot of the same properties of firmware-to-
> >>> Linux boot formats: It needs a stable, documented ABI that allows for
> >>> forward and backward compatibility as well as versioning.
> >>
> >> Please use subject prefixes matching the subsystem. You can get them for
> >> example with `git log --oneline -- DIRECTORY_OR_FILE` on the directory
> >> your patch is touching. For bindings, the preferred subjects are
> >> explained here:
> >> https://www.kernel.org/doc/html/latest/devicetree/bindings/submitting-patches.html#i-for-patch-submitters
> >  
> > These are not devicetree binding for communicating data from firmware to
> > the kernel. These bindings are specific to KHO which is perfectly
> > reflected by the subject.
> 
> No, it is not. None of the bindings use above subject prefix.
> 
> > 
> > Just a brief reminder from v2 discussion:
> > (https://lore.kernel.org/linux-mm/20231222193607.15474-1-graf@amazon.com/)
> > 
> > "For quick reference: KHO is a new mechanism this patch set introduces 
> > which allows Linux to pass arbitrary memory and metadata between kernels 
> > on kexec. I'm reusing FDTs to implement the hand over protocol, as 
> > Linux-to-Linux boot communication holds very similar properties to 
> > firmware-to-Linux boot communication. So this binding is not about 
> > hardware; it's about preserving Linux subsystem state across kexec.
> 
> does not matter. You added file to ABI documentation so you must follow
> that ABI documentation rules. One rule is proper subject prefix.
 
No, it does not. It's a different ABI.

FDT is a _data structure_ that provides cross platform unified, versioned,
introspectable data format.

Documentation/devicetree/bindings standardizes it's use for describing
hardware, but KHO uses FDT _data structure_ to describe state of the kernel
components that will be reused by the kexec'ed kernel.

KHO is a different namespace from Open Firmware Device Tree, with different
requirements and different stakeholders. Putting descriptions of KHO data
formats in Documentation/kho rather than in
Documentation/devicetree/bindings was not done to evade review of Open
Firmware Device Tree maintainers, but rather to emphasize that KHO FDT _is
not_ Open Firmware Device Tree.

> Best regards,
> Krzysztof

-- 
Sincerely yours,
Mike.