linux-kernel - Re: [PATCH v2 17/17] devicetree: Add bindings for ftrace KHO

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Message-ID: <b860528c-d6b9-4531-8522-acbadc154b15@kernel.org>
Date: Sun, 24 Dec 2023 09:58:57 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Alexander Graf <graf@...zon.com>, linux-kernel@...r.kernel.org
Cc: linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org,
 devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 kexec@...ts.infradead.org, linux-doc@...r.kernel.org, x86@...nel.org,
 Eric Biederman <ebiederm@...ssion.com>, "H. Peter Anvin" <hpa@...or.com>,
 Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Rob Herring <robh+dt@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Mark Rutland <mark.rutland@....com>, Tom Lendacky <thomas.lendacky@....com>,
 Ashish Kalra <ashish.kalra@....com>, James Gowans <jgowans@...zon.com>,
 Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>, arnd@...db.de,
 pbonzini@...hat.com, madvenka@...ux.microsoft.com,
 Anthony Yznaga <anthony.yznaga@...cle.com>,
 Usama Arif <usama.arif@...edance.com>, David Woodhouse <dwmw@...zon.co.uk>,
 Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v2 17/17] devicetree: Add bindings for ftrace KHO

On 24/12/2023 00:20, Alexander Graf wrote:
>>> new file mode 100644
>>> index 000000000000..9960fefc292d
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/kho/ftrace/ftrace-array.yaml
>>> @@ -0,0 +1,46 @@
>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>>> +%YAML 1.2
>>> +---
>>> +$id: http://devicetree.org/schemas/kho/ftrace/ftrace-array.yaml#
>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>>> +
>>> +title: Ftrace trace array
>>> +
>> Missing description. Commit msg also does not tell me much. This must
>> stand on its own and must describe the hardware. Whatever you have in
>> cover letter, does not matter, especially that you did not Cc us on it.
> 
> 
> Alrighty, I'll add descriptions and make the commit message stand on its 
> own.
> 
> 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.

Devicetree is for non-discoverable systems and their hardware, not for
passing arbitrary data between kernels. For me this does not suit DT at
all, please use other ways.


> 
> For more details, please refer to the KHO documentation which is part of 
> patch 7 of this patch set: 
> https://lore.kernel.org/lkml/20231222195144.24532-2-graf@amazon.com/
> 
> 
>>
>>> +maintainers:
>>> +  - Alexander Graf <graf@...zon.com>
>>> +
>>> +properties:
>>> +  compatible:
>>> +    enum:
>>> +      - ftrace,array-v1
>>> +
>>> +  trace_flags:
>> Underscores are not allowed. Does not look like generic property.
> 
> 
> Let me make it "trace-flags" to not have underscores. Could you please 
> elaborate on what you mean by generic property?

Generic property, so one without vendor prefix, is shared and common to
a group of devices.


> 
> 
>>
>>
>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>> +    description:
>>> +      Bitmap of all the trace flags that were enabled in the trace array at the
>>> +      point of serialization.
>>> +
>>> +# Subnodes will be of type "ftrace,cpu-v1", one each per CPU
>>> +additionalProperties: true
>> No, this must be false. And it goes after required:
> 
> 
> Ok, making it false and adding pattern matches instead for subnodes.
> 
> 
>>
>>
>>> +
>>> +required:
>>> +  - compatible
>>> +  - trace_flags
>>> +
>>> +examples:
>>> +  - |
>>> +    ftrace {
>>> +        compatible = "ftrace-v1";
>>> +        events = <1 1 2 2 3 3>;
>>> +
>>> +        global_trace {
>> Again, no underscores.
> 
> 
> Ok :)
> 
> 
>>
>>> +          compatible = "ftrace,array-v1";
>>> +          trace_flags = < 0x3354601 >;
>>> +
>>> +          cpu0 {
>>> +            compatible = "ftrace,cpu-v1";
>>> +            cpu = < 0x00 >;
>> Drop redundant spaces.
> 
> 
> I don't understand what you're referring to as redundant spaces? Double 
> checking, I believe indentation is off for every line below "ftrace {". 
> Is that what you're referring to? Fixing :)

Open DTS, some recent, arm64 like Qualcomm. Do you see spaces around <>?
Or open the coding style document... Please do not introduce different
coding style.

> 
> 
>>
>>> +            mem = < 0x101000000ULL 0x38ULL 0x101000100ULL 0x1000ULL 0x101000038ULL 0x38ULL 0x101002000ULL 0x1000ULL>;
>> ? Do you see any of such syntax in DTS?
> 
> 
> I was trying to make it easy to reason to readers about 64bit numbers - 

64bit numbers are not a problem for DTS reading. Above syntax is.

> and then potentially extend dtc to consume that new syntax. KHO DTs are 
> native/little endian, so dtc already has some difficulties interpreting 
> it which I'll need to fix up with patches to it eventually :). I'll 
> change it to something that looks more 32bit'y for now.
> 


Best regards,
Krzysztof