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Date:   Sat, 6 Mar 2021 23:58:55 +0100
From:   Thomas Bogendoerfer <tsbogend@alpha.franken.de>
To:     Rob Herring <robh@kernel.org>
Cc:     Paul Cercueil <paul@crapouillou.net>, od@zcrc.me,
        "open list:MIPS" <linux-mips@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] MIPS: boot/compressed: Copy DTB to aligned address
Message-ID: <20210306225855.GA3574@alpha.franken.de>
References: <20210303193305.924384-1-paul@crapouillou.net>
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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@alpha.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@crapouillou.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 ?
> 
> There was a long discussion on devicetree-compiler list a few months
> ago. In summary, a while back libfdt switched to accessors from raw
> pointer accesses to avoid any possible unaligned accesses (is MIPS
> always okay with unaligned accesses?).

no, it will trap unaligned accesses, that's the reason for Paul's problem.

> This was determined to be a
> performance regression and an overkill as the DT structure itself
> should always be naturally aligned if the dtb is 64-bit aligned. I
> think 32-bit aligned has some possible misaligned accesses.

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

> 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.

Thomas.

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