ACPI vs DT at runtime

[Olof Johansson]

On Mon, Nov 18, 2013 at 12:33:19PM -0500, Jon Masters wrote:
> On 11/18/2013 03:45 AM, Arnd Bergmann wrote:
> > On Sunday 17 November 2013, Olof Johansson wrote:
> 
> >> Yep, and together for review is a critical part. I'm not saying that
> >> the ideal solution is a flashed DTB, but it's better than ACPI. A
> >> flashed DTB that's _easy to update_ as part of an OS install would
> >> probably be one of the best solutions we could ask for though.
> > 
> > Right: the assumption on the DT boot interfaces is that board designers
> > make mistakes and are working on moving targets, so we need ways to update
> > the DTB. By contrast, the assumption on ACPI is that server firmware
> > authors know what they are doing and you shouldn't let anyone else mess
> > with the ACPI bits because doing so might damage your hardware. We're
> > in trouble if the second assumption turns out wrong.
> 
> In the server space we *never* ship updated ACPI tables in Red Hat
> products, and we never would ship updated tables as part of the OS.
> Platform data is platform data. It belongs to the vendor shipping the
> system, never to the OS. Yes, there are workarounds for bugs but on the
> whole you do in fact count on server vendors not to screw it up, and if
> they do then it is up to them to ship a BIOS update for their hardware.

And in x86 land these days, they mostly don't screw it up because they've been
doing this for years and years. On ARM we've got a dozen new vendors who have
never done any of this before (and neither have we), and we're all going to
screw it up. Denying that buys us nothing, planning for it in a manner that we
can recover from the worst screwups without paying for it forever is the best
way forward.

> In the Fedora space, even on 32-bit, we tried to not get in the business
> of shipping DeviceTree blobs (way back when) that should be shipped by
> the platform, but alas many systems don't have flash to store the DTB.

Well, duh, that was what we, as the kernel community, had mandated at
the time.

> Coupled with the moving target we're dealing with there...things have
> been ugly. In the v8 server space, there will be no shipping of platform
> data. It will be ACPI tables passed in via UEFI, never updated by the
> OS, and owned wholly by the platform vendor.

Everything is a moving target on ARMv8 today. ACPI isn't going to change that.

-Olof