Re: [Buildroot] [RFC PATCH 1/2] support/scripts/genimage.sh: support creating a bmap image

From: Thomas Petazzoni via buildroot <buildroot@buildroot.org>
To: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Dario Binacchi <dario.binacchi@amarulasolutions.com>,
	Xuanhao Shi <X15000177@gmail.com>,
	buildroot@buildroot.org, Romain Naour <romain.naour@smile.fr>,
	Michael Nazzareno Trimarchi <michael@amarulasolutions.com>,
	linux-amarula@amarulasolutions.com,
	Anand Gadiyar <gadiyar@ti.com>
Subject: Re: [Buildroot] [RFC PATCH 1/2] support/scripts/genimage.sh: support creating a bmap image
Date: Fri, 31 May 2024 11:44:53 +0200	[thread overview]
Message-ID: <20240531114453.59ccda1a@windsurf> (raw)
In-Reply-To: <ZkwwN1NKZ2o1xMba@landeda>

On Tue, 21 May 2024 07:25:11 +0200
"Yann E. MORIN" <yann.morin.1998@free.fr> wrote:

> > Arnout, Yann, Peter, Romain, what do you think? Do you see other
> > approaches to easily allow generating bmap-compatible images?  
> 
> I wonder if we should really do that, in fact.
> 
> I believe it is in-scope that we do generate disk images, which we do
> with genimage. However, transferring, distributing, writing, etc.. the
> generated images I think is out of scope. We may provide tools to help
> with that, though, and we do by providing {host-,}bmap-tools.

I think it is in the scope of Buildroot to have defconfigs that
generate images that are fast to flash.

> I think the question by Thomas was whether, given an input sparse file,
> genimage will keep the generated image sparse in the same places the
> input file was (only shifted by the offset that input file was copied
> at in the image).
> 
> For example, mkfs.ext can create sparse files, and such a file can be
> fed to genimage, to create a disk image. We want the generated disk
> image to be sparsed in the same positions the ext2 image was, e.g. with
> 'X' as no-hole blocks, '.' as holes, and 'B' as the bootloader:
> 
>   rootfs.ext2        XXXXX........XXX...XXXX....XXXXX........X....
>   disk.img       BBBBXXXXX........XXX...XXXX....XXXXX........X....
> 
> Note that some X block can be all-zeroes, because they are the bitmap of
> the filesystem; those must not be skipped when writing to the final
> storage, otherwise the filesystem would be corrupted. If genimage
> decides that all-zeroes block are sparsed, then this is broken.
> 
> I think that was the question raised by Thomas, and I don't understand
> how your reply answers that, so maybe I missed something...

This was exactly my question, indeed.

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com
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