[Letux-kernel] Subtle issue with U-Boot and kernel and SD card UUIDs

H. Nikolaus Schaller hns at goldelico.com
Tue May 23 12:02:01 CEST 2017

Hi Marek,

> Am 23.05.2017 um 11:25 schrieb Belisko Marek <marek.belisko at gmail.com>:
> Hi Nikoalus,
> On Tue, May 23, 2017 at 10:42 AM, H. Nikolaus Schaller
> <hns at goldelico.com> wrote:
>> Hi,
>> I recently flashed the eMMC of the OMAP5EVM and the Pyra and now a subtle issue
>> has surfaced.
>> If I boot without SD card from eMMC everything is fine.
>> But if I boot from the SD card it works fine for U-Boot and loading the kernel,
>> but the kernel loaded from SD card mounts the rootfs from eMMC and does not find
>> any loadable modules (unless kernels are exactly the same version).
>> I could trace the u-boot-kernel interaction down to the uboot environment
>>        finduuid=part uuid mmc ${mmcdev}:2 uuid
>>        args_mmc=run finduuid;setenv bootargs console=${console} ${optargs} root=PARTUUID=${uuid} rw rootfstype=${mmcrootfstype}
> I was facing similar issue yesterday but my root looks like
> root=LABEL=rootfsA. I just drop LABEL=rootfsA and replace by /dev/sdb2
> (in my case) and it works fine.

Yes, this is the "old style" way of defining the root by a hard-coded name.

But the problem is that there is no guarantee any more in modern kernels
that sdb2 is not sdc2 on next boot. Depending on which other devices
are plugged in.

And I want the bootargs to somehow be the same if I boot from eMMC or SD
card. Or we have to ask users to configure uEnv.txt each time they plug
in different cards...

As long as the device just has one device of each type it is stable.

> I was trying shortly look to kernel source and I cannot find anything
> related to LABEL but for your case it should work (please loon for
> comment for function name_to_dev_t in ini/do_mounts.c)
> Hope this helps.

The problem with LABEL is that they all have the same label by the way
the makesd script is doing the partitioning. It creates a "boot" and one
"rootfs" partition. And then we have the same problem: the eMMC provides
a "rootfs" partition as well as the SD card that is plugged in.

And, I can't make the label match the numbering scheme U-Boot is assuming
(mmc 0:2 vs. mmc 1:2).

The UUID approach was invented to solve all these issues. But for that it
should work...

>> This is not at all wrong and should do what it is intended for: map the mmc
>> device numbers from U-Boot in a way that the kernel can assign a different
>> order (and it does!).
>> This is done by translating the device numbers into a unique code, i.e. the
>> UUID.
>> Now why does it not work in my setup?
>> Breaking into the U-Boot console shows the problem:
>> => part list mmc 0
>> Partition Map for MMC device 0  --   Partition Type: DOS
>> Part    Start Sector    Num Sectors     UUID            Type
>>  1     128             778233          00000000-01     0c Boot
>>  2     778361          14786439        00000000-02     83
>> => part list mmc 1
>> Partition Map for MMC device 1  --   Partition Type: DOS
>> Part    Start Sector    Num Sectors     UUID            Type
>>  1     128             386656          00000000-01     0c Boot
>>  2     386784          7346464         00000000-02     83
>> =>
>> Both, the eMMC and the SD card have the same UUIDs i U-Boot and therefore
>> the kernel picks up the wrong one...
>> Now how can they have the same UUID? It should be unique, shouldn't it?
>> That is strange since after booting into Linux (still without
>> kernel modules), I can look up how Linux sees the storage partitions:
>> root at letux:~# blkid
>> /dev/mmcblk0p1: LABEL="boot" UUID="E052-71A6" TYPE="vfat"
>> /dev/mmcblk0p2: LABEL="rootfs" UUID="5db8fefe-f8d2-46e4-828a-ab4fe1b8f9b4" TYPE="ext3"
>> /dev/mmcblk1p1: LABEL="boot" UUID="ED66-B684" TYPE="vfat"
>> /dev/mmcblk1p2: LABEL="rootfs" UUID="cd5f3d4c-c562-4aa0-893c-9fa9e9bd64a9" TYPE="ext3"
>> /dev/mmcblk0: PTTYPE="dos"
>> /dev/mmcblk1: PTTYPE="dos"
>> root at letux:~#
>> And there, we have nice UUIDs!
>> Looks as if U-Boot does not recognize them. But the kernel still can mount
>> some rootfs by "root=PARTUUID=00000000-02". Strange.

I have digged a little into the U-Boot code and the 'part list' command


calls part_print().

There appear to be different part_print subfunctions for each partition scheme.

iMac:src hns$ fgrep part_print -R System.map 
System.map:8001b084 T part_print
System.map:8001b744 T part_print_dos
System.map:8001bc58 t part_print_iso
System.map:8001c118 T part_print_efi
iMac:src hns$ 

And since I format the SD cards by MBR format, I guess part_print_dos() is called.


This finally calls 


There it simply prints %08x-%02x which corresponds to our PARTUUID=00000000-02

So it appears as if U-Boot is not capable of detecting the full UUID of an ext3 partition on a MBR disk while the kernel does.

What I have seen while going through the code: there is some CONFIG_PARTITION_TYPE_GUID
Maybe we need to define it?


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