linux-kernel - Re: [PATCH v8 08/21] dt / chosen: Add linux,uefi-stub-generated-dtb property

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Message-ID: <CAKv+Gu-oDRjYPC3SxSDVZ7rV7eG9DCct=kNS0xBVd28YoV5fSg@mail.gmail.com>
Date:	Sat, 7 Feb 2015 05:03:44 +0000
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Hanjun Guo <hanjun.guo@...aro.org>
Cc:	G Gregory <graeme.gregory@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Mark Langsdorf <mlangsdo@...hat.com>,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	"wangyijing@...wei.com" <wangyijing@...wei.com>,
	Rob Herring <robh@...nel.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Jonathan Corbet <corbet@....net>,
	Timur Tabi <timur@...eaurora.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Charles Garcia-Tobin <Charles.Garcia-Tobin@....com>,
	"phoenix.liyi@...wei.com" <phoenix.liyi@...wei.com>,
	Robert Richter <rric@...nel.org>,
	Jason Cooper <jason@...edaemon.net>,
	Arnd Bergmann <arnd@...db.de>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"jcm@...hat.com" <jcm@...hat.com>, Mark Brown <broonie@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Ashwin Chaugule <ashwinc@...eaurora.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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	Sudeep Holla <Sudeep.Holla@....com>,
	Olof Johansson <olof@...om.net>
Subject: Re: [PATCH v8 08/21] dt / chosen: Add linux,uefi-stub-generated-dtb property

On 7 February 2015 at 03:36, Hanjun Guo <hanjun.guo@...aro.org> wrote:
> On 2015年02月06日 18:34, G Gregory wrote:
> [...]
>
>>>>>>
>>>>>> --------------------------------------------------------------------------------
>>>>>>   linux,uefi-stub-kern-ver  | string | Copy of linux_banner from
>>>>>> build.
>>>>>>
>>>>>> --------------------------------------------------------------------------------
>>>>>> +linux,uefi-stub-generated-dtb  | bool | Indication for no DTB
>>>>>> provided by
>>>>>> +                        |      | firmware.
>>>>>>
>>>>>> +--------------------------------------------------------------------------------
>>>>>
>>>>>
>>>>> Apologies for the late bikeshedding, but the discussion on this topic
>>>>> previsously was lively enough that I thought I'd let it die down a bit
>>>>> before seeing if I had anything to add.
>>>>>
>>>>> That, and I just realised something:
>>>>> One alternative to this added DT entry is that we could treat the
>>>>> absence of a registered UEFI configuration table as the indication
>>>>> that no HW description was provided from firmware, since the stub does
>>>>> not call InstallConfigurationTable() on the DT it generates. This does
>>>>> move the ability to detect to after efi_init(), but this should be
>>>>> fine for ACPI-purposes.
>>>>>
>>>> That would not work as expected in the kexec/Xen use case though as they
>>>> may genuinely boot with DT from an ACPI host without UEFI.
>>>
>>>
>>> I'm a little concerned by this case. How do we intend to pass stuff from
>>> Xen to the kernel in this case? When we initially discussed the stub
>>> prior to merging, we weren't quite sure if ACPI without UEFI was
>>> entirely safe.
>>>
>>> The linux,uefi-stub-kern-ver property was originally intended as a
>>> sanity-check feature to ensure nothing (including Xen) masqueraded as
>>> the stub, but for some reason the actual sanity check was never
>>> implemented.
>>>
>>>>> If that is deemed undesirable, I would still prefer Catalin's
>>>>> suggested name ("linux,bare-dtb"), which describes the state rather
>>>>> than the route we took to get there.
>>>>>
>>>> I agree.
>>>
>>>
>>> I guess this would be ok, though it would be nice to know which agent
>>> generated the DTB.
>>>
>>
>> The most obvious scheme then is
>>
>> linux,bare-dtb = "uefi-stub";
>>
>> otherwise we generate a new binding for every component in the boot path.
>
>
> Leif, Mark, any comments on this?
>

As far as I remember, we did not finalize the decision to go with a
stub generated property instead of some other means to infer that the
device tree is not suitable for booting and ACPI should be preferred.

We will be discussing the 'stub<->kernel interface as a boot protocol'
topic this week at Connect, so let's discuss it in that context before
signing off on patches like these.

-- 
Ard.
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