linux-kernel - Re: [PATCH] mtd: allow mtd and jffs2 when ARCH=um

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Date:	Tue, 14 Dec 2010 22:12:28 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	dedekind1@...il.com
Cc:	Jason Lunz <lunz@....org>,
	richard -rw- weinberger <richard.weinberger@...il.com>,
	Sam Ravnborg <sam@...nborg.org>,
	David Woodhouse <dwmw2@...radead.org>,
	atom ota <atomota@...epyhammer.com>,
	user-mode-linux-devel@...ts.sourceforge.net,
	Jeff Dike <jdike@...toit.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mtd@...ts.infradead.org, Rob Landley <rob@...dley.net>
Subject: Re: [PATCH] mtd: allow mtd and jffs2 when ARCH=um

On Tue, Dec 14, 2010 at 21:01, Artem Bityutskiy <dedekind1@...il.com> wrote:
> On Tue, 2010-12-14 at 11:51 -0800, Jason Lunz wrote:
>> On Tue, Dec 14, 2010 at 06:24:38PM +0200, Artem Bityutskiy wrote:
>> > But I think your solution is a bit dirty, because it adds a great deal
>> > of little 'if HAS_IOMEM' and '#ifdef CONFIG_HAS_IOMEM' to many places.
>> > This is error-prone.
>>
>> The intent of that patch was to allow as much of the mtd subsystem to
>> compile as possible. My thinking was to try and rectify the fact that
>> uml has gone without mtd (and hence jffs2) support for years even though
>> much of it works just fine. I think the entire subsystem being marked
>> BROKEN in kconfig kept anyone from experimenting with it.
>>
>> The patch I sent was actually a reaction to feedback I got from Sam
>> Ravnborg on my last attempt (um, three years ago :/ ) in which he
>> suggested pushing down the ifdefs closer to their points of use. But I
>> agree, the minimal version has a much smaller footprint.
>>
>> The version below still meets the goal of allowing jffs2-on-block2mtd
>> usage under uml but is much smaller because only the mtd core is
>> included. Compile-tested on i386, x86_64, um/i386, and um/x86_64.
>>
>> > Instead, you should solve this problem in UML code. I do not know how,
>> > but may be you can add readb/writeb there which actually do nothing or
>> > print a scary warning, or do BUG(), and let things which use them just
>> > fail run-time.
>>
>> Something like this could work, but it would be error-prone for anyone
>> else who attempts using iomem-requiring drivers on uml. Instead of
>> getting obvious compile failures we'd have broken drivers that BUG() or
>> emit scary warnings. That doesn't seem to me like an improvement.
>
> This problem does not seem to be mtd-specific, right? So my point was
> that it would be nicer to come up with a general solution.

The generic solution for stuff that needs I/O operations is to mark it
"depends on HAS_IOMEM".

I used a similar patch in the past, IIRC to try axfs on UML.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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