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Message-ID: <01b82f6eb37b674effc6c8b0fa4a014deb401a85.camel@infradead.org>
Subject: Re: [PATCH] jffs2: implement mount option to configure endianness
From:   David Woodhouse <dwmw2@infradead.org>
To:     Richard Weinberger <richard.weinberger@gmail.com>, nkela@cisco.com
Cc:     "linux-mtd @ lists . infradead . org" <linux-mtd@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, xe-linux-external@cisco.com
Date:   Thu, 08 Nov 2018 09:11:54 +0100
In-Reply-To: <CAFLxGvz8QgK1P6d-_biSRk4fyYEobio315_tUP+cRXTvt+m1hA@mail.gmail.com>
References: <20181106214928.40020-1-nkela@cisco.com>
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         <591D4732-BC3E-4F85-9277-25E049FFF4BA@cisco.com>
         <CAFLxGvz8QgK1P6d-_biSRk4fyYEobio315_tUP+cRXTvt+m1hA@mail.gmail.com>
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On Wed, 2018-11-07 at 19:14 +0100, Richard Weinberger wrote:
> On Wed, Nov 7, 2018 at 7:05 PM Nikunj Kela (nkela) <nkela@cisco.com> wrot=
e:
> > I had tried to use configs to start with via the following patch howeve=
r I was advised to have a mount option:
> > http://lists.infradead.org/pipermail/linux-mtd/2018-November/085126.htm=
l
>=20
> Just show performance numbers on how your implementation has an impact or=
 not.
> So far your implementation is also not much optimized, maybe likely()
> or static keys can help...

Using likely() for the native case might help. Static keys might help a
little more, but could only work if every file system has the *same*
endianness. Unless we end up with three variants, for native vs. swap
vs. runtime checking.

We also lose a bunch of the optimisations that we gained from using
__builtin_swab functions, which let the compiler see what was going on.

But we can hypothesise and handwave about it until the cows come home;
I'd like to see a real test of whether it actually makes a difference
that we care about.

If it does, one option might be to just build separate versions of
scan.c for each endianness, since that's the critical path we care
about.

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