linux-kernel - Re: [PATCH] jffs2: implement mount option to configure endianness

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Date:   Thu, 08 Nov 2018 09:11:54 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Richard Weinberger <richard.weinberger@...il.com>, nkela@...co.com
Cc:     "linux-mtd @ lists . infradead . org" <linux-mtd@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, xe-linux-external@...co.com
Subject: Re: [PATCH] jffs2: implement mount option to configure endianness

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@...co.com> wrote:
> > I had tried to use configs to start with via the following patch however I was advised to have a mount option:
> > http://lists.infradead.org/pipermail/linux-mtd/2018-November/085126.html
> 
> 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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