AW: How to overwrite an ext partition on eMMC

[Martin Hollingsworth <Martin.Hollingsworth@itk-engineering.de>]

Hi Sascha,
huge thanks, that was just the right push in the right direction. Both suggested methods - cp and memcpy - work fine. Problem solved.

> Why don't you use the partitions from the partition table on the device?
> I would assume you use /dev/mmc3.2 for the rootfs.

I assume you are referring to using "detect -a" to find the device, which is then registered as mmc3.0 (for first partition). As I was struggling to write the root.ext2 onto that device I decided to try using the devfs_add_partition() method, as all nand flash based devices I have as reference use this approach. However I just verified, using the detected partition works as well and it is probably the better way of doing it, as the partitions can be moved around more easily.

In another side question you write: "I would assume you use /dev/mmc3.2 for the rootfs". As I currently only copy the Barebox raw onto the eMMC offset 0x400 for iMX6 it is not listed as partition under /dev/. What is the standard approach here, should the Barebox best be a FAT32 partition so it is listed?

Thanks and regards,
Martin

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-----Ursprüngliche Nachricht-----
Von: Sascha Hauer [mailto:s.hauer@pengutronix.de] 
Gesendet: Dienstag, 5. Juli 2016 08:55
An: Martin Hollingsworth
Cc: barebox@lists.infradead.org
Betreff: Re: How to overwrite an ext partition on eMMC

On Mon, Jul 04, 2016 at 12:32:04PM +0000, Martin Hollingsworth wrote:
> Hello folks,
> I'm a little lost when trying to overwrite an ext partition on an eMMC 
> memory inside of barebox. Your help finding the mistake is 
> appreciated.
> 
> My Setup:
> - Custom board with iMX6, 4GB eMMC and SD card reader (similar to 
> Freescale SabreSD board)
> - Using PTXdist to build barebox 2016.05.0 and linux
> - The eMMC chip offers wear levelling, so I write a filesystem 
> directly to it (using ptxdist created hd.img file flashed directly)
> - The eMMC is partitioned as follows:
> 0x0, Size 1k --> partition table
> 0x400, Size 8M --> barebox and barebox_env (offset 0x400 forced by 
> iMX6) 0x800400, Size 1G --> ext filesystem with rootfs and kernel
> 
> With this layout so far everything works fine. Now I would like to 
> implement an update mechanism, where barebox erases the complete ext 
> partition and replaces it. Under linux I would use something like dd 
> and let it start at 0x800400. On barebox I have to use memcpy (thanks 
> to Sascha for the hint 
> http://lists.infradead.org/pipermail/barebox/2011-April/003308.html ) 
> and this is where I get stuck.
> 
> So I first add partitions so that the memory area is listed under /dev:
> devfs_add_partition("mmc3", 0x0, SZ_1K, DEVFS_PARTITION_FIXED, 
> "mmc3.partable"); c("mmc3", SZ_1K, SZ_8M, 
> DEVFS_PARTITION_FIXED, "mmc3.barebox"); devfs_add_partition("mmc3", ( 
> SZ_1K + SZ_8M ), SZ_1G, DEVFS_PARTITION_FIXED, "mmc3.rootfs");

Why don't you use the partitions from the partition table on the device?
I would assume you use /dev/mmc3.2 for the rootfs.

> 
> This works for clearing the partitions data using memset:
> memset -d /dev/mmc3.rootfs 0x0 0x0 1073741824
> 
> However when I try to copy the root.ext2 filesystem onto this memory area, I can't mount the partition afterwards:
> memcpy -s /mnt/sd/root.ext2 -d /dev/mmc3.rootfs 0 536870912

memcpy needs <src> <dest> <count> positional arguments. With the above
536870912 is the offset in the destination file and not the size to copy. What you want is:

memcpy -s /mnt/sd/root.ext2 -d /dev/mmc3.rootfs 0 0 536870912

Anyway, you don't need memset/memcpy at all to accomplish your task, the following should do it:

cp /dev/zero /dev/mmc3.rootfs
cp /mnt/sd/root.ext2 /dev/mmc3.rootfs

Also I have never seen that it's necessary to erase the remaining parts of a partition when the new image is smaller than the partition.

Sascha

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