[mdlug] UEFI Booting [Resolved]

[A. Zimmer]

On Tue, 04 Nov 2014 12:30:25 -0600
Bob <bob at starlinephoto.com> wrote:

> 
> Not sure you need EFI turned on in the bios or in the kernel to
> read/write GPT partitions.
> 

I found the answer. CONFIG_EFI_PARTITION=y is necessary, but something
else also is needed.

During kernel configuration, when selecting the "Advanced partition selection"
option, a whole slew of sub-options is revealed.  One of these sub-options
is CONFIG_EFI_PARTITION and is a necessary choice.  But another sub-option
is CONFIG_MSDOS_PARTITION, which enables access to the ordinary MBR partitions.
This must also be selected (if one wants to access both MBR *and* GPT).

Since my system has never used the kernel "Advanced partition selection"
I just assumed that access to MBR partitions was the built-in default
and consequently I failed to set CONFIG_MSDOS_PARTITION=y.  My system
was then unable to boot with an ordinary MBR formatted disk.

It is a subtle detail, but if missed it will cause serious problems (and
all of the available documents and tutorials on Linux GPT/UEFI *do* miss
it).

Anyway, now I have access to both MBR *and* GPT partitions.