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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.03.2023 09:37, Roger Pau Monné wrote:
> 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 think so, yes.

> 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.

Or maybe when the set mode isn't text-output capable.

> 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?

Of course there is. As said, earlier on screens started in 80x25 mode.
Even going to 80x50 or 80x60 is already an improvement. Plus there are
systems which support wider-than-80-cols text modes.

Jan