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Date:   Wed, 10 Feb 2021 21:55:48 +0100
From:   Miquel Raynal <miquel.raynal@bootlin.com>
To:     Richard Weinberger <richard@nod.at>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Boris Brezillon <boris.brezillon@collabora.com>,
        Ron Minnich <rminnich@google.com>, sven <sven@narfation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-mtd <linux-mtd@lists.infradead.org>,
        fuse-devel <fuse-devel@lists.sourceforge.net>
Subject: Re: [PATCH 0/8] MUSE: Userspace backed MTD v3
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Hi Richard,

Richard Weinberger <richard@nod.at> wrote on Wed, 10 Feb 2021 12:23:53
+0100 (CET):

> Miquel,
> 
> ----- Ursprüngliche Mail -----
> >> Does in-band and OOB data need to be handled together?  
> > 
> > Short answer: yes.
> >   
> >> If so, then two requests is not a good option.  
> > 
> > More detailed answer:
> > 
> > There is a type of MTD device (NAND devices) which are composed, for
> > each page, of X in-band bytes plus Y out-of-band metadata bytes.
> > 
> > Accessing either the in-band data, or the out-of-band data, or both at
> > the same time are all valid use cases.
> > 
> > * Read operation details:
> >  From a hardware point of view, the out-of-band data is (almost)
> >  always retrieved when the in-band data is read because it contains
> >  meta-data used to correct eventual bitflips. In this case, if both
> >  areas are requested, it is highly non-efficient to do two requests,
> >  that's why the MTD core allows to do both at the same time.
> > * Write operation details:
> >  Even worse, in the write case, you *must* write both at the same
> >  time. It is physically impossible to do one after the other (still
> >  with actual hardware, of course).
> > 
> > That is why it is preferable that MUSE will be able to access both in
> > a single request.  
> 
> By single request we meant FUSE op-codes. The NAND simulator in Userspace
> will see just one call. My plan is to abstract it in libfuse.

If libfuse abstracts it, as long as MTD only sees a single request I'm
fine :)

Thanks,
Miquèl