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

[Joakim Tjernlund <Joakim.Tjernlund@infinera.com>]

On Thu, 2018-11-08 at 18:01 +0000, Nikunj Kela (nkela) wrote:
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> 
> On 11/8/18, 12:12 AM, "David Woodhouse" <dwmw2@infradead.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@cisco.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