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

[Hans de Goede]

Hi,

On 18-06-18 10:43, Ard Biesheuvel wrote:
> On 18 June 2018 at 10:30, Hans de Goede <hdegoede at redhat.com> wrote:
>> 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?
> 
> UEFI does not usually clear the screen when it boots the default
> entry, so it is entirely dependent on the boot loader that runs next.
> I guess GRUB usually clears the screen unconditionally?

It depends, GRUB either leaves it completely alone (leaving the
BGRT graphics which the firmware hopefully has put up already
in place) or if it actually draws anything, then it clears iit
before starting the kernel.

> In any case, I think it is reasonable to clear the screen if you
> redraw the logo, but I do wonder if it is safe to assume that the
> background color is black.
> 
> Instead, we may decide to clear the screen before ExitBootServices()
> [using EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL.ClearScreen()].

On most x86 machines (but not all, GRR) the firmware itself will
draw the logo already. I actually have grub patches pending to make
it not do any text-output related calls at all unless it actually
has a message / menu it wants to display.

Calling EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL.ClearScreen() will cause
a bootup sequence like this:

firmware draws logo
clearscreen
redraw logo

Which will cause an ugly flash to black. Where as the purpose
is to have a smooth boot with the logo being on screen all
the time without any flickers / flashes.

I've never seen a machine where the background is not black,
the background not being black would also break the spinning
dots which microsofts draws on top of the background while
booting, so a memset to 0 seems sensible to me.

>>   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.
>>
> 
> I think the overhead of doing the clearing is not going to be the
> bottleneck. But redrawing a logo on black background that was designed
> to be displayed over another color is going to look really ugly ...

Do you know of any examples of this ?

There seems to be no known way to get the background color, so black /
all 0 seems the be the best bet.

I would expect any non black background logos to simply be screen
filling.

Regards,

Hans