[PATCH v1] doc: README.distro: Special case with Windows formatted disk

[Pali Rohár]

On Friday 17 January 2020 12:44:51 Andy Shevchenko wrote:
> If someone wants to use shared (by installed OS) eMMC partition to
> the Windows to boot from, it's not possible due to U-Boot limitations.
> 
> Describe this case and possible workaround.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
> ---
>  doc/README.distro | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/doc/README.distro b/doc/README.distro
> index ab6e6f4e74..807a82c910 100644
> --- a/doc/README.distro
> +++ b/doc/README.distro
> @@ -405,3 +405,23 @@ of the boot environment and are not guaranteed to exist or work in the same
>  way in future u-boot versions.  In particular the <device type>_boot
>  variables (e.g. mmc_boot, usb_boot) are a strictly internal implementation
>  detail and must not be used as a public interface.
> +
> +Using a eMMC partition that has been formatted as a disk by Windows 10
> +======================================================================
> +
> +Let's assume we have an (embedded) board with U-Boot and Linux OS
> +installed on eMMC. Linux OS shares one of the eMMC partitions as
> +a disk via USB Mass Storage protocol.
> +
> +It may be useful to utilize that disk to copy bootable files from
> +Windows machine to the board in case someone doesn't want to erase
> +stock installation on it.
> +
> +Unfortunately, Windows 10 doesn't provide knobs and always formats
> +that disk as a whole, meaning that it creates a partition table on it
> +with requested (FAT) partition. As a result U-Boot may not see any
> +files on it due to nesting partition tables.
> +
> +The workaround may be in formatting the partition under Linux OS,
> +setting up a network connection between Linux OS and Windows 10 and
> +use it to copy files to the partition.

There is a better way how to do it. You can format partition to FAT with
fake MBR table which reference to itself. Windows can correctly
recognize such disk because it has MBR table and do not care that
partition in MBR table starts at same offset as MBR table itself. And
FAT filesystem can be put on such partition because first sector (where
is stored MBR) is not used by FAT itself. So non-FAT data can be stored
there. Also Linux does not have any issue to access such partition
because filesystem starts at offset zero (where it should be).

FAT fs can be formatted with this fake MBR table by "mformat" tool which
should work also on Windows systems. "mformat" supports this feature for
a longer time but I do not remember how to activate it. Passing correct
arguments to "mformat" always took me lot of time.

Or alternatively it can be formatted by "mkfs.fat" tool with option
--mbr=y but support for this option is not in any released version of
"mkfs.fat" yet.