About text mode on EFI when booting Linux

About text mode on EFI when booting Linux

[Dennis Chen]

Hi Vladimir,

We encounter an error message "no suitable video mode found" when
booting a VM on i386 platform. Though I think this is not a big problem,
but the LinuxKit guys(we use grub2 in LinuxKit project) have some
concerns about this error message, so I submit a patch[1] try to
eliminate this message. Would you please take a look at that patch?

I also dig into the commit logs of the grub2 git tree, and found below
commit:

commit b1257f653367e8d21b625276f53f086d68fab0b3
Author: Vladimir 'phcoder' Serbinenko <phcoder@gmail.com>
Date:   Tue Aug 16 16:11:10 2011 +0200

        Don't accept text modes on EFI when booting Linux.
    
        * grub-core/loader/i386/linux.c (ACCEPTS_PURE_TEXT): New define.
        (grub_linux_boot) [!ACCEPTS_PURE_TEXT]: Restrict to graphics modes. 

...
diff --git a/grub-core/loader/i386/linux.c b/grub-core/loader/i386/linux.c
index 9e3d482..5356d7a 100644
--- a/grub-core/loader/i386/linux.c
+++ b/grub-core/loader/i386/linux.c
@@ -45,15 +45,18 @@ GRUB_MOD_LICENSE ("GPLv3+");
 #include <grub/efi/efi.h>
 #define HAS_VGA_TEXT 0
 #define DEFAULT_VIDEO_MODE "auto"
+#define ACCEPTS_PURE_TEXT 0
 #elif defined (GRUB_MACHINE_IEEE1275)
 #include <grub/ieee1275/ieee1275.h>
 #define HAS_VGA_TEXT 0

I am very curious about the context/background of this commit here, what's
the reason leads us not to accept a text modes on EFI when booting Linux?

[1] https://lists.gnu.org/archive/html/grub-devel/2017-10/msg00031.html

Thanks,
Dennis 

Re: About text mode on EFI when booting Linux

[Matthias Lange]

Hi,

On 2017-1109 at 17:49:53 +0800, Dennis Chen wrote:
> Hi Vladimir,
> 
> We encounter an error message "no suitable video mode found" when
> booting a VM on i386 platform. Though I think this is not a big problem,
> but the LinuxKit guys(we use grub2 in LinuxKit project) have some
> concerns about this error message, so I submit a patch[1] try to
> eliminate this message. Would you please take a look at that patch?
> 
> I also dig into the commit logs of the grub2 git tree, and found below
> commit:
>
[...]
> 
> I am very curious about the context/background of this commit here, what's
> the reason leads us not to accept a text modes on EFI when booting Linux?

it appears that (U)EFI dos not feature any text services. See also this thread
[1] from the help-grub mailing list.

As far as I have understood things, UEFI has the "Console I/O Protocol" API
which can be used to output to a console without caring about what the console
actually is (serial port or something). To output something on the screen the
"Graphics Output Protocol" is used which doesn't support any text mode.

Best,
Matthias.

[1] https://lists.gnu.org/archive/html/help-grub/2011-12/msg00023.html

> [1] https://lists.gnu.org/archive/html/grub-devel/2017-10/msg00031.html
> 
> Thanks,
> Dennis