linux-kernel - Re: [PATCH 1/4] Documentation: arm: [U]EFI runtime services

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Message-ID: <CACxGe6s-TZoNUaRZ+XpAC83OGPWeg41L9F7YHYjpXVeLR6vbEw@mail.gmail.com>
Date:	Thu, 27 Jun 2013 21:11:45 +0100
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Leif Lindholm <leif.lindholm@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, linux-efi@...r.kernel.org,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>, matt.fleming@...el.com
Subject: Re: [PATCH 1/4] Documentation: arm: [U]EFI runtime services

On Thu, Jun 27, 2013 at 7:04 PM, Stephen Warren <swarren@...dotorg.org> wrote:
> On 06/26/2013 01:31 PM, Leif Lindholm wrote:
>> On Wed, Jun 26, 2013 at 12:32:30PM -0600, Stephen Warren wrote:
>>>>> What about ARMv8? Is the intention to have a separate definition for the
>>>>> UEFI bindings on ARMv8, so that compatibility isn't an issue? What if a
>>>>> future version of UEFI allows LPAE usage?
>>>>
>>>> It is unlikely that will happen on v7 since newer versions of UEFI are
>>>> expected to remain backwards compatible with the current spec.
>>>
>>> The expectation of backwards-compatibility sounds nice, but it seems a
>>> little dangerous to outright rely on it.
>>>
>>> Even if not a regular compatible property, can we define a property that
>>> indicates the UEFI revision or revision of this DT binding, so that if
>>> we ever have to change it, there is some way of explicitly indicating
>>> which exact schema the DT corresponds to, rather than having to
>>> reverse-engineer it from the set of properties that "just happen" to be
>>> present in DT?
>>>
>>> This is rather like the firmware node discussion that happened recently,
>>> where we were expecting to represent a firmware (secure mode) interface
>>> by a DT node, which would have a compatible value, which in turn would
>>> convey information about which "OS" the secure firmware was running, and
>>> well as any potential SoC-/OEM-/board-specific interface provided by it.
>>>
>>> And who knows, what if UEFI gets replaced someday; presumably we'd want
>>> some way of explicitly stating "running under UEFI" vs. "running under
>>> something else"?
>>
>> To me, these concerns are all covered by the existence of the
>> efi-system-table node, and the version number that you can extract
>> from the table (mandatory in any UEFI implementation) located at that
>> address. There is no reverse-engineering involved.
>
> So, what you're saying is that the existence (or lack thereof) of the
> efi-system-table property is the indicator whether EFI is present? I
> guess if we assume that EFI will always evolve at least compatibly
> enough that the system table will always exist and be formatted
> identically at least to the extent of allowing the EFI version number to
> be parsed then that's workable. If that guarantee is broken, then we can
> always define a different property that points at the new format of the
> table.

Yes, that is what it means, and there is *immense* pressure from the
OEM market to not break that contract in a non-backwards-compatible
way.

g.
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