[PATCH 2/2] efifb: Copy the ACPI BGRT boot graphics to the framebuffer

[Hans de Goede]

Hi,

On 18-06-18 09:36, Ard Biesheuvel wrote:
> Hallo Hans,
> 
> On 17 June 2018 at 17:32, Hans de Goede <hdegoede at redhat.com> wrote:
>> On systems where fbcon is configured for deferred console takeover, the
>> intend is for the framebuffer to show the boot graphics (e.g a vendor
>> logo) until some message (e.g. an error) is printed or a graphical
>> session takes over.
>>
>> Some firmware however relies on the OS to show the boot graphics
>> (indicated by bgrt_tab.status being 0) and the boot graphics may have
>> been destroyed by e.g. the grub boot menu.
>>
>> This patch adds support to efifb to show the boot graphics and
>> automatically enables this when fbcon is configured for deferred
>> console takeover.
>>
>> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> 
> I have tested this code on ARM QEMU/mach-virt, and with a little tweak
> (which I will post separately), the code works as expected, i.e., it
> redraws the boot logo based on the contents of the BGRT table.

That is great.

> However, what it doesn't do is clear the screen, which means the logo
> is drawn on top of whatever the boot environment left behind, and I
> end up with something like this.
> 
> http://people.linaro.org/~ard.biesheuvel/mach-virt-bgrt-logo-redrawn.png

Hmm, less great. I'm not sure how to deal with this, on x86 it is more
or less guaranteed that the screen is already cleared when we load and
clearing a 4k screen means writing about 32MB, which I guess with modern
RAM speeds is not that bad actually.

I see that you got this picture by manual booting from the EFI shell,
in what state does the firmware / bootloader normally leave the
framebuffer?  I'm asking because if normally it is either cleared
to black, or already showing the logo I wonder if we should take the
(small) penalty of clearing ?

Given that we are talking about only 32 MB I could do a v2 which just
memsets the rest of the screen to 0.

So we get:

	for (y= 0; y < height; y++) {
		if (line_part_of_bgrt) {
			memset(left-of-bgrt);
			draw_bgrt_line(y);
			memset(right-of-bgrt);
		} else {
			memset(line);
		}
	}

Note I've deliberately done the if on a per line
base to keep the actual blit part of the loop
efficient and without any extra conditionals in
there. I also don't simply first memset the entire
fb to 0 to avoid a flash / tearing if the bgrt
image is already in place (which happens often on
x86).

Implementing this is easy and as said the extra execution time should
be quite small, still I wonder what others think about this?

I'm leaning towards doing the clearing / memsets since I've seen
some firmwares leave some artifacts from not completely clearing
things like a "Press F2 to enter setup" message, missing a few
pixels and leaving those on screen.

Regards,

Hans