BBB eMMC boot partition

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1

Hello,

I’m new here, so I’ll introduce myself, I’m an embedded software developer working with ARM based boards (A10, iMx, AM335x) from the Netherlands.

I’ve been working on a custom linux installation for the BBB (buildroot). There’s only one nagging problem I can’t seem to solve. For booting I use u-boot and an uSD card with three partitions. The first is FAT. This work fine. But then comes the problem: eMMC.

It seems like the only way to let the BBB boot off the eMMC is to create a FAT partition (#1) that goes from cylinder 1 to 9 (255 heads, 63 sectors). The eMMC is big enough for sure, so that’s no problem, but it bothers me that I can’t seem to change the partition size, and I don’t understand why.

On the partition is a vfat filesystem with the MLO, u-boot.img and uEnv.txt files.

I’ve read the AM335x technical reference thoroughly, checked all the boot flowcharts against a hexdump of the mmcblk1 device, but I can’t find the reason why the device won’t boot when I change the partition size.

Does anyone know about any requirements I’ve missed? Is the size a given constraint? I would like to use a very small FAT partition of 1MB or so…

Thanks,

Sjef.

2

Hello,

I'm new here, so I'll introduce myself, I'm an embedded software developer
working with ARM based boards (A10, iMx, AM335x) from the Netherlands.

I've been working on a custom linux installation for the BBB (buildroot).
There's only one nagging problem I can't seem to solve. For booting I use
u-boot and an uSD card with three partitions. The first is FAT. This work
fine. But then comes the problem: eMMC.

It seems like the only way to let the BBB boot off the eMMC is to create a
FAT partition (#1) that goes from cylinder 1 to 9 (255 heads, 63 sectors).
The eMMC is big enough for sure, so that's no problem, but it bothers me
that I can't seem to change the partition size, and I don't understand why.

That's an old requirement, it was a "x-loader" bug, that got fixed
years ago.. (x-loader (original MLO) couldn't find "u-boot.bin")

On the partition is a vfat filesystem with the MLO, u-boot.img and uEnv.txt
files.

I've read the AM335x technical reference thoroughly, checked all the boot
flowcharts against a hexdump of the mmcblk1 device, but I can't find the
reason why the device won't boot when I change the partition size.

Does anyone know about any requirements I've missed? Is the size a given
constraint? I would like to use a very small FAT partition of 1MB or so..

fat16 needs 12MB or greater to work.

btw, instead of bothering with the fat partition, just use one big ext4:

sudo dd if=MLO of=/dev/sdX count=1 seek=1 conv=notrunc bs=128k
sudo dd if=u-boot.img of=/dev/sdX count=2 seek=1 conv=notrunc bs=384k

Then just remember, to create the first partition with a 1mb hole at the start.

sudo sfdisk --in-order --Linux --unit M ${DISK} <<-__EOF__
1,0x83,*
__EOF__

Regards,

3

Hello,

I’m new here, so I’ll introduce myself, I’m an embedded software developer
working with ARM based boards (A10, iMx, AM335x) from the Netherlands.

I’ve been working on a custom linux installation for the BBB (buildroot).
There’s only one nagging problem I can’t seem to solve. For booting I use
u-boot and an uSD card with three partitions. The first is FAT. This work
fine. But then comes the problem: eMMC.

It seems like the only way to let the BBB boot off the eMMC is to create a
FAT partition (#1) that goes from cylinder 1 to 9 (255 heads, 63 sectors).
The eMMC is big enough for sure, so that’s no problem, but it bothers me
that I can’t seem to change the partition size, and I don’t understand why.

That’s an old requirement, it was a “x-loader” bug, that got fixed
years ago… (x-loader (original MLO) couldn’t find “u-boot.bin”)

Thing is, MLO isn’t even loaded, I just get the CCCC console output.

On the partition is a vfat filesystem with the MLO, u-boot.img and uEnv.txt
files.

I’ve read the AM335x technical reference thoroughly, checked all the boot
flowcharts against a hexdump of the mmcblk1 device, but I can’t find the
reason why the device won’t boot when I change the partition size.

Does anyone know about any requirements I’ve missed? Is the size a given
constraint? I would like to use a very small FAT partition of 1MB or so…

fat16 needs 12MB or greater to work.

I din’t realize that… I have a 1MB FAT32 partition on the uSD card that works just fine…
So I reasoned that this should work on the eMMC too.,.

btw, instead of bothering with the fat partition, just use one big ext4:

sudo dd if=MLO of=/dev/sdX count=1 seek=1 conv=notrunc bs=128k
sudo dd if=u-boot.img of=/dev/sdX count=2 seek=1 conv=notrunc bs=384k

Then just remember, to create the first partition with a 1mb hole at the start.

sudo sfdisk --in-order --Linux --unit M ${DISK} <<-EOF
1,0x83,*
EOF

This is a good solution, thank you. It still bothers me that I can’t get a handle
on why the FAT booting only works with a single specific partition size, but
I’ll let it go :slight_smile:

Best regards,

Sjef.