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

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Message-ID: <CAJfpegvN2KdMj_7T-OF1PAs8xZiU3f4233AvigaXwwRAsgQEjw@mail.gmail.com>
Date:   Tue, 9 Feb 2021 15:26:25 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Richard Weinberger <richard@....at>
Cc:     Miquel Raynal <miquel.raynal@...tlin.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Ron Minnich <rminnich@...gle.com>, sven@...fation.org,
        linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
        fuse-devel <fuse-devel@...ts.sourceforge.net>
Subject: Re: [PATCH 0/8] MUSE: Userspace backed MTD v3

On Mon, Jan 25, 2021 at 12:21 AM Richard Weinberger <richard@....at> wrote:
>
> I'm happy to announce the first non-RFC version of this patch set.
> Over the xmas holidays I found some time to experiment with various userspace
> implementations of MTDs and gave the kernel side more fine-tuning.
>
> Rationale:
> ----------
>
> When working with flash devices a common task is emulating them to run various
> tests or inspect dumps from real hardware. To achieve that we have plenty of
> emulators in the MTD subsystem: mtdram, block2mtd, nandsim.
>
> Each of them implements an ad-hoc MTD and have various drawbacks.
> Over the last years some developers tried to extend them but these attempts
> often got rejected because they added just more adhoc feature instead of
> addressing overall problems.
>
> MUSE is a novel approach to address the need of advanced MTD emulators.
> Advanced means in this context supporting different (vendor specific) image
> formats, different ways for fault injection (fuzzing) and recoding/replaying
> IOs to emulate power cuts.
>
> The core goal of MUSE is having the complexity on the userspace side and
> only a small MTD driver in kernelspace.
> While playing with different approaches I realized that FUSE offers everything
> we need. So MUSE is a little like CUSE except that it does not implement a
> bare character device but an MTD.

Looks fine.

I do wonder if MUSE should go to drivers/mtd/ instead.   Long term
goal would be move CUSE to drivers/char and move the transport part of
fuse into net/fuse leaving only the actual filesystems (fuse and
virtiofs) under fs/.

But for now just moving the minimal interface needed for MUSE into a
separate header (<net/fuse.h>) would work, I guess.

Do you think that would make sense?

>
> Notes:
> ------
>
> - OOB support is currently limited. Currently MUSE has no support for processing
>   in- and out-band in the same MTD operation. It is good enough to make JFFS2
>   happy. This limitation is because FUSE has no support more than one variable
>   length buffer in a FUSE request.
>   At least I didn’t find a good way to pass more than one buffer to a request.
>   Maybe FUSE folks can correct me. :-)

If you look at fuse_do_ioctl() it does variable length input and
output at the same time.  I guess you need something similar to that.

Thanks,
Miklos