Re: [PATCH] ArmVirtPkg: remove QemuVideoDxe from ArmVirtQemu and ArmVirtQemuKernel

[Leif Lindholm <leif.lindholm@linaro.org>]

On Wed, Aug 23, 2017 at 03:36:37PM +0200, Laszlo Ersek wrote:
> On 08/23/17 15:17, Ard Biesheuvel wrote:
> >>>> (My R-b stands; these are comments for a possible followup patch.)
> >>>>
> >>>> Please see:
> >>>>
> >>>> - commit 84a75f70e903 ("OvmfPkg/QemuVideoDxe: enable ARM builds",
> >>>> 2015-02-23),
> >>>>
> >>>> - commit 05a537945872 ("OvmfPkg/QemuVideoDxe: Helper functions for
> >>>> unaligned port I/O.", 2017-04-07)?
> >>>>
> >>>> In my opinion, we should now revert parts of these commits, in one
> >>>> followup patch:
> >>>>
> >>>> - from the first commit, we should revert only the "VALID_ARCHITECTURES"
> >>>> comment change (the rest is built upon by the second commit, and should
> >>>> be preserved)
> >>>>
> >>>> - from the second commit, we should revert the addition of [Sources.ARM,
> >>>> Sources.AARCH64].
> >>>>
> >>>> This boils down to removing ARM and AARCH64 references from the
> >>>> QemuVideoDxe.inf file. If you agree, could you please submit such a
> >>>> followup patch?
> >>>
> >>> Sure, but pending the graphical GRUB discussion.
> >>
> >> So, after looking at the GRUB code, I am leaning towards agreeing that
> >> this is actually not a problem at all ... probably. The efi_gop driver
> >> does a Blt() of the entire screen from an off-screen buffer for all
> >> updates _unless_ it fails to allocate that off-screen buffer.
> >>
> >> So, basically, if you run out of memory at that point, it will try to
> >> preserve a way to get messages out about that. I will send a question
> >> out to grub-devel regarding this behaviour.
> >>
> >> However, looking at the specification, a question remains over how
> >> software can determine whether direct FB access is possible. I mean, a
> >> value of 0 seems like a decent hint, but the spec says nothing on the
> >> topic.
> >>
> > 
> > It will assume the FB is accessible unless the pixel format is PixelBltOnly
> > 
> 
> Correct, the UEFI spec (v2.7) says in "12.9 Graphics Output Protocol":
> 
>   PixelBltOnly  This mode does not support a physical frame buffer.
> 
> and
> 
>   PixelFormat   Enumeration that defines the physical format of the
>                 pixel. A value of PixelBltOnly implies that a linear
>                 frame buffer is not available for this mode.

-ETOOMANYLEVELSOFINDIRECTION
Right, so that's a non-issue.

Hopefully, that means other operating systems can also deal with the
lack.

> IIRC simply recognizing and accepting this enum constant was the point
> of Alex's patch.

Ah, yes, that makes sense now.
It's a bit surprising things work without this patch, but GRUBs
fallback seems to match anyway.

/
    Leif