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

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Date:   Thu, 8 Nov 2018 18:50:32 +0000
From:   Joakim Tjernlund <Joakim.Tjernlund@...inera.com>
To:     "richard.weinberger@...il.com" <richard.weinberger@...il.com>,
        "nkela@...co.com" <nkela@...co.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "xe-linux-external@...co.com" <xe-linux-external@...co.com>
Subject: Re: [PATCH] jffs2: implement mount option to configure endianness

On Thu, 2018-11-08 at 18:01 +0000, Nikunj Kela (nkela) wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.
> 
> 
> 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?

I think that is a good idea. 

 Jocke