Re: [PATCH 2/5] efi: only set a console mode if the current one is invalid

["Roger Pau Monné" <roger.pau@citrix.com>]

On Fri, Mar 31, 2023 at 08:51:46AM +0200, Jan Beulich wrote:
> On 30.03.2023 18:17, Roger Pau Monné wrote:
> > On Thu, Mar 30, 2023 at 06:07:57PM +0200, Jan Beulich wrote:
> >> On 30.03.2023 17:44, Roger Pau Monné wrote:
> >>> I guess I'm slightly confused by the usage of both GOP and StdOut, I
> >>> would assume if we have a gop, and can correctly initialize it there's
> >>> no need to fiddle with StdOut also?
> >>
> >> Setting the GOP mode is done last before exiting boot services; this
> >> may be a graphics mode which doesn't support a text output protocol.
> > 
> > Right, that's what I was missing.  I assumed that all modes available
> > in GOP would be compatible with the ConOut mode.
> > 
> > Would you be OK with leaving StdOut as-is when booted from multiboot2,
> > or there's a chance of things not being properly setup?
> 
> On modern UEFI it may be unlikely, but I think it's not impossible (see
> below).
> 
> > IMO it's not very friendly to change the StdOut mode if not explicitly
> > requested, as in the multiboot2 case that gets setup by the
> > bootloader.
> 
> May get set up, that is. If it was set up, then yes, we probably should
> leave it alone unless told to use another mode. I.e. no vga= or
> vga=current should minimally result in no further mode change. Aiui we
> can't easily honor vga=gfx-... in that case, so leaving the mode alone
> there may also be better than trying to guess a mode. The only time
> where I would think it would be nice to switch by default even in the
> xen.gz case is if the boot loader handed us the screen in some text
> mode.

How would you detect such case?

ConOut is always text-mode like because it's a
EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL interface.

Would it be a matter of checking whether the current GOP mode is
valid, and if so leave it as-is unless told otherwise by a command
line parameter?

I would also like to avoid the unconditional resizing of the ConOut
interface that's done in efi_console_set_mode(), as that has the size
effect of changing the GOP mode, so I would only call
efi_console_set_mode() is there's no gop.

Not sure it's meaningful to change the ConOut number of cols/rows if
there's no GOP, maybe it's possible to have some kind of screen that's
usable for EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL but not as a GOP?

Thanks, Roger.