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To: linux-kernel@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: Mounting a in-ROM filesystem efficiently
Date: 13 Dec 2001 12:38:28 -0800
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Copyright: Copyright 2001 H. Peter Anvin - All Rights Reserved
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Followup to:  <20011213160007.D998D23CCB@persephone.dmz.logatique.fr>
By author:    Thomas Capricelli <orzel@kde.org>
In newsgroup: linux.dev.kernel
> 
> Hello,
> 
> I'm looking for a way to put a filesystem into ROM.
> Seems pretty trivial, isn't it ?
> 
> My understanding is (the way initrd does, and the way I do as of today)
> * create a RAMDISK
> * loads the data into ramdisk
> * mount the ramdisk
> 
> problem is that I don't want to waste the RAM as the data in the ROM is 
> already in the address space. (it's an embedded system, btw)
> 
> Speed is not an issue here. ROM access might be slower than RAM, it will 
> always be so much quicker than a disk access. (wrong?)
> 

Frequently wrong.  Depending on the interface, ROM can be just
unbelievably slow.

> Ideally, i would give address/length of the fs in ROM to a function, and I 
> would get a ramdisk configured to read its data exactly there, and not in 
> ram.

The right thing for you to do is to write a block device driver, and
then mount that block device like any order device.  Your in-use data
will be copied to RAM (i.e. cached), but it can be dropped and
re-fetched as necessary.  This should be the desired behaviour.

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
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