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

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Message-ID: <CAL_JsqK0_M18gnoYFyTyf_OaQgbmbYYyoAr-WaFCzzsmFuFeFg@mail.gmail.com>
Date:   Sat, 6 Mar 2021 14:35:21 -0700
From:   Rob Herring <robh@...nel.org>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
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 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 ?

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?). 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.

As part of this, a dtb alignment check was added. So worst case, we
could disable that if need be.

Rob