fat: handle Windows formatted partition (thru USB Mass Storage)

[Heinrich Schuchardt]

On 1/16/20 11:39 AM, Andy Shevchenko wrote:
>
>>> Obviously U-Boot's fat code cannot handle it.
>>
>> So precisely, U-Boot cannot handle nested partition( table)s?
>
> Seems so. We need to be able to supply the partition number we would
> like to open, something like
>    cmd <interface> [<dev>[:<partition>[:<nested partition>]]]
> otherwise it will require some (error prone) heuristics to understand
> which one user would like to use.
>

I have formatted a USB stick Windows 10.

When I display the partition table with fdisk it shows:

Device     Boot      Start        End    Sectors   Size Id Type
/dev/sdb1       1935758368 3615603091 1679844724   801G 61 SpeedStor
/dev/sdb2                0          0          0     0B 65 Novell
Netware 386
/dev/sdb4         28049408   28049850        443 221.5K  0 Empty

But there is no partition table at all. The FAT filesystem starts at
sector 0. I can mount the disk with

sudo mount /dev/sdb /mnt

In U-Boot I can read the file system as partition 0:

=> ls scsi 0:0
             System Volume Information/
        11   test.txt.txt

1 file(s), 1 dir(s)

=> load scsi 0:0 0x40200000 test.txt.txt
11 bytes read in 10 ms (1000 Bytes/s)
=> md.b 0x40200000 b
40200000: 48 65 6c 6c 6f 20 77 6f 72 6c 64   Hello world
=>

The UEFI shell started from U-Boot sees the file system:

Mapping table
       FS0: Alias(s):F0a0:;BLK0:
           /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/Scsi(0,0)

In the UEFI shell I created a new text file. I could read it in Windows.

Now I formatted the USB in Linux with gdisk to give it a GPT partition
table:

Number  Start (sector)    End (sector)  Size       Code  Name
    1            2048        33556479   16.0 GiB    0700  Microsoft
basic data
    2        33556480        62521310   13.8 GiB    0700  Microsoft
basic data

and formatted the partitions with mkfs.vfat as FAT32.

Now Windows sees this USB stick as two partions and can read files
created in Linux.

So this is what I learnt:

Windows 10 does not support MBR partition tables on USB sticks.
Windows 10 supports GPT partition tables on USB sticks.
Windows formats empty USB sticks without any partition table at all.
There is no obvious compatibility issue in U-Boot.

Best regards

Heinrich