Re: legacy-bios and uefi boot usb stick

[Andrei Borzenkov]

14.05.2017 16:25, John Frankish пишет:
>>>>> I've made a couple of dual legacy-bios/uefi boot usb sticks using 
>>>>> grub-2.00.
>>>>>
>>>>> Both of the usb sticks will uefi boot linux, but there appears to be a 
>>>>> problem with legacy-bios boot of linux.
>>>>>
>>>>> One of the usb sticks gives an "invalid partition table!" error
>>>>>
>>>> What displays the message ? GRUB or the BIOS firmware ? Do you get the 
>>>> GRUB rescue prompt ?
>>>> Some BIOS firmwares require that a primary partition has the boot flag.
>>>>
>>> It seems as though it is grub displaying the message.
>>>
>>> In one case I get the "invalid partition table!" message and have to 
>>> cycle the power
>>>
>> If the message was displayed by GRUB, I believe that you would at least get 
>> the GRUB rescue prompt.
>>
> After videoing the screen, I see that the error message is:
> 
> Error: file '/EFI/BOOT/grub/i386-pc/efi_gop.mod' not found
> Error: no suitable video modes found
> 
> efi_gop* is in i386-efi and x86_64-efi, but not in i386-pc.
> 

Which is correct, this video driver is available on EFI platform only.

> If I add "vga=792" then grub gives an error message that this is depreciated, 
> but the frame buffer is set to 1024x768. If I try "vga=895" the native 
> resolution of 1366x768 is not set even though it is present in "Xvesa 
> -listmodes" and is set with uefi boot.
> 

It is not clear what "is not set" means here (is not set in grub or in
Linux), but your grub.cfg in original post does not load any video
driver for for legacy BIOS.

In current grub you can simply use "insmod all_video" which will attempt
to load all available video drivers. Usually only one of them actually
works. This should handle both legacy BIOS and EFI.