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

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Message-ID: <B1D7CC34-C9D3-4745-94A1-3F713C1CB216@cisco.com>
Date:   Thu, 8 Nov 2018 18:01:23 +0000
From:   "Nikunj Kela (nkela)" <nkela@...co.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Richard Weinberger <richard.weinberger@...il.com>
CC:     "linux-mtd @ lists . infradead . org" <linux-mtd@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "xe-linux-external(mailer list)" <xe-linux-external@...co.com>
Subject: Re: [PATCH] jffs2: implement mount option to configure endianness


On 11/8/18, 12:12 AM, "David Woodhouse" <dwmw2@...radead.org> wrote:

    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.

I wonder if this feature is really that important that we need to duplicate the drivers.
Also, it might take some time for me to find some device that I can run the tests with and without this patch.
I am wondering if we can still consider my first patch with config options as a good compromise on it?