[syslinux] Equivalent to GRUB_GFXPAYLOAD_LINUX=keep with vesamenu?

[nerdopolis]

On Tuesday, March 16, 2021 11:59:26 PM EDT Gregory Bartholomew wrote:
> On Tue, Mar 16, 2021 at 9:07 PM nerdopolis <bluescreen_avenger at verizon.net>
> wrote:
> 
> > ...
> > I could do a VGA= option, but with GRUB_GFXPAYLOAD_LINUX=keep you don't
> > have to
> > pass a VGA= option for a framebuffer to be initialized. It's not a kernel
> > parameter, but a bootloader wide option, to not switch back to text mode
> > while
> > booting the kernel
> >
> > Since I am doing this on an ISO, I want to make as little assumptions
> > about the
> >
> > hardware as possible, so I am trying to see if there is a way I can ensure
> > a
> > framebuffer on all cards, including the 'vmware' card that QEMU has.
> > With GRUB, it initializes a framebuffer with the
> > GRUB_GFXPAYLOAD_LINUX=keep
> > option. Otherwise, including with ISOLINUX, it does not initialize a
> > framebuffer, and I read that it's the bootloader that initializes the
> > framebuffer for the kernel usually.
> >
> >
> >
> > Is there an option to make syslinux/isolinux not return to text mode when
> > booting a kernel? Or is this not possible?
> >
> > Thanks
> >
> >
> >
Hi
I am just realizing I mistakenly replied to you directly instead of the mailing
list. I am sorry about that. 
> According to
> https://lists.syslinux.org/archives/syslinux-commits/2010-January/000027.html,
> syslinux should leave the video mode alone by default.
> 
> I'm not an expert on this, but I think all GRUB_GFXPAYLOAD_LINUX=keep does
> is write "0x0f04" to the two bytes at offset 01FA (see here:
> https://www.kernel.org/doc/Documentation/x86/boot.txt). It is just a
> round-about way of setting vga=current without actually specifying it on
> the kernel command line. If your video mode is changing, I think it is the
> kernel that is changing it, not syslinux. You'll need to tell the kernel,
> somehow, not to change the video mode. If you have something like Plymouth
> installed in the initramfs, that could also be the culprit. Maybe make sure
> "splash" and/or "rhgb" aren't specified on the command line.
> 
OK, I am realizing that in syslinux there IS a special case for 
vga=current / vga=
https://repo.or.cz/syslinux.git/blob/346d2278e23f1b9f02903506753263ad7226de58:/com32/lib/syslinux/load_linux.c#l497

Trying that option I do see much different behavior. It doesn't flush the
display, as if though going into text mode (which is a good start), but it
appears The kernel never creates the /dev/fb0 device like it does with grub
starting it.

I didn't know about that 0x0f04 option before as well, so that's cool.


If I have to continue to hardcode resolution options, that is fine. I had that
in these ISOs for quite a bit. 

The reason why I wanted to try to better guarantee a frame-buffer, is that if
modesetting doesn't work on these ISOs, it falls back to Weston's framebuffer
backend, as Weston, not X11 is the default greeter.

And I am reading of a Linux mode setting driver that is in the works that might
rely on the Framebuffer space to act as a limited /dev/dri/card0 device
> I think there are also differences between BIOS and UEFI with regard to how
> framebuffers are handled. See for example
> https://cateee.net/lkddb/web-lkddb/FB_EFI.html and
> https://forums.gentoo.org/viewtopic-t-1066642-start-0.html.
> 
> Your best bet for a "one size fits all" solution is probably to set a very
> standard/compatible resolution in syslinux (e.g. "menu resolution 800 600")
> and then set vga=current and nomodeset on the kernel command line. I think
> a lot of ISOs are distributed with multiple boot menu options that offer
> "fallback" video modes precisely because a "universal" video mode across
> all hardware doesn't really exist (even less so right now because of the
> BIOS to UEFI transition).
> 

Thanks