linux-kernel - Re: [PATCH] MIPS: boot/compressed: Copy DTB to aligned address

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Message-ID: <20210308174523.GA13680@alpha.franken.de>
Date:   Mon, 8 Mar 2021 18:45:23 +0100
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Rob Herring <robh@...nel.org>
Cc:     Paul Cercueil <paul@...pouillou.net>, od@...c.me,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: boot/compressed: Copy DTB to aligned address

On Mon, Mar 08, 2021 at 10:04:15AM -0700, Rob Herring wrote:
> On Sat, Mar 6, 2021 at 3:59 PM Thomas Bogendoerfer
> <tsbogend@...ha.franken.de> wrote:
> >
> > On Sat, Mar 06, 2021 at 02:35:21PM -0700, Rob Herring wrote:
> > > On Sat, Mar 6, 2021 at 1:45 AM Thomas Bogendoerfer
> > > <tsbogend@...ha.franken.de> wrote:
> > > >
> > > > On Wed, Mar 03, 2021 at 02:37:55PM -0600, Rob Herring wrote:
> > > > > On Wed, Mar 3, 2021 at 1:33 PM Paul Cercueil <paul@...pouillou.net> wrote:
> > > > > >
> > > > > > Since 5.12-rc1, the Device Tree blob must now be properly aligned.
> > > > >
> > > > > I had checked the other built-in cases as microblaze broke too, but
> > > > > missed some of the many ways MIPS can have a dtb. Appended and
> > > > > built-in DTBs were supposed to be temporary. :(
> > > >
> > > > and a fdt can also be provided by firmware. And according to spec
> > > > there is no aligmnet requirement. So this whole change will break
> > > > then. What was the reason for the whole churn ?
> 
> Actually, that is wrong. The spec defines the alignment (from
> flattened format appendix):

I was talking about the "Unified Hosting Interface" from MIPS/Imagination.
As the spec talks about device tree blob all firmware developer knew
about the fdt alignment rules.

> > the access macros are using *(unsigned long long *), which isn't
> > even nice for 32bit CPUs...
> 
> Where are those?

nowhere, I've missread the code in libfdt_env.h

> > > As part of this, a dtb alignment check was added. So worst case, we
> > > could disable that if need be.
> >
> > yeah, or override fdt32/64_to_cpu, if I understood the code correctly.
> 
> No, fdt32/64_to_cpu don't dereference the pointer.

you are right, brainfart on my side.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]