[U-Boot] [PATCH 14/18] efi: stub: Pass EFI GOP information to U-Boot payload

[Bin Meng]

Hi Alex,

On Mon, Jun 11, 2018 at 3:34 PM, Alexander Graf <agraf at suse.de> wrote:
> On 06/11/2018 08:01 AM, Bin Meng wrote:
>>
>> Hi Alex,
>>
>> On Mon, Jun 11, 2018 at 1:52 PM, Alexander Graf <agraf at suse.de> wrote:
>>>
>>>
>>> On 11.06.18 01:29, Bin Meng wrote:
>>>>
>>>> On Mon, Jun 11, 2018 at 3:16 AM, Alexander Graf <agraf at suse.de> wrote:
>>>>>
>>>>>
>>>>> On 10.06.18 15:25, Bin Meng wrote:
>>>>>>
>>>>>> If UEFI BIOS has the graphics output protocol (GOP), let's pass its
>>>>>> information to U-Boot payload so that U-Boot can utilize it (eg:
>>>>>> an EFI framebuffer driver).
>>>>>>
>>>>>> Signed-off-by: Bin Meng <bmeng.cn at gmail.com>
>>>>>
>>>>> Why can't the FB drive determine all of this on its own and just fail
>>>>> probe if no GOP protocol can be found?
>>>>>
>>>> It cannot. Once U-Boot payload is running, the boot services are gone.
>>>> There is no way to determine the GOP protocol.
>>>
>>> Interesting. Is there a particular reason you're not preserving boot
>>> services?
>>>
>> This is EFI payload support with CONFIG_EFI_STUB. Preserving boot
>> services is EFI application, with CONFIG_EFI_APP. For example, see
>> serial_efi.c which is the serial driver that uses EFI's boot services
>> to output characters on the serial port.
>
>
> Oh, I see. That makes sense now.
>
> Do people actually need CONFIG_EFI_STUB then?

I think you wanted to say: Do people actually need CONFIG_EFI_APP
then? CONFIG_EFI_STUB is more useful than CONFIG_EFI_APP. The
application support is very limited.

As the README.u-boot_on_efi says:

Running U-Boot on EFI is useful in several situations:

- You have EFI running on a board but U-Boot does not natively support it
fully yet. You can boot into U-Boot from EFI and use that until U-Boot is
fully ported

- You need to use an EFI implementation (e.g. UEFI) because your vendor
requires it in order to provide support

- You plan to use coreboot to boot into U-Boot but coreboot support does
not currently exist for your platform. In the meantime you can use U-Boot
on EFI and then move to U-Boot on coreboot when ready

- You use EFI but want to experiment with a simpler alternative like U-Boot

Right now, I have one Intel SkyLake board, and before I get a native
U-Boot up and running as the 1st stage bootloader on this board, I can
run U-Boot as the EFI payload to experiment something, which is very
handy.

>
>>
>>> It might be quite handy to reuse UEFI block interfaces for example. It
>>> might also be useful to preserve (parts of?) the memory map so we don't
>>> overwrite random SMM code for example.
>>>
>>> Maybe I misunderstand the purpose of the UEFI U-Boot support though ;).
>>
>> Looks there is some misunderstanding around the EFI payload support.
>
>
> Yes, I have to admit that I never really cared enough about it to read its
> documentation though :).

Yep, this sometimes confuses people :)

Regards,
Bin