Re: grub cannot boot from ext4 on mmc ("unknown filesystem")

[Andrei Borzenkov]

On 18.07.2022 15:37, Andreas Ufert wrote:
> Hi,
> here's my setup:
> 
> grub 2.06
> build with yocto/kirkstone
> 
> grub (arch i386-pc) is installed on (hd0) (MBR) and (hd0,msdos1) (modules 
> and config)
> kernel to bee booted resides on (hd0,msdos2)
> 
> grub fails with "error: unknown filesystem"
> 
> grub *can read* (hd0,msdos1) (vfat filesystem):
> 
>       ls (hd0,msdos1)/
>       grub/
>       grub>
> 
> grub *can see* (hd0,msdos2), but *cannot detect* a filesystem:
> 
>       ls (hd0,msdos2)
>       Partition hd0,msdos2: No known filesystem detected - Partition start at 
> 33792KiB - Total size 2097152KiB
> 
> grub *cannot read* (hd0,msdos2)/:
> 
>       ls (hd0,msdos2)/
>       error: unknown filesystem
> 
> 
> All relevant modules are loaded, particularly ext2.
> 
> 
> If I dd the very same image onto an USB key grub perfectly reads the ext4 
> filesystem and boots the kernel so one could argue that MMC is the problem. 
> However, the vfat partition can be read from MMC just fine. It's only the 
> MMC/ext4 combination that fails somehow.
> 

Did you try to format the same partition alternatively as FAT and ext4
and check access?

grub is using BIOS services to access devices so it is possible that
BIOS has issues accessing partition "too far" as example.