linux-kernel - Re: [PATCH 0/8] MUSE: Userspace backed MTD v3

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Date:   Wed, 10 Feb 2021 12:23:53 +0100 (CET)
From:   Richard Weinberger <richard@....at>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Vignesh Raghavendra <vigneshr@...com>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Ron Minnich <rminnich@...gle.com>, sven <sven@...fation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        fuse-devel <fuse-devel@...ts.sourceforge.net>
Subject: Re: [PATCH 0/8] MUSE: Userspace backed MTD v3

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.

Thanks,
//richard