linux-kernel - Re: efi/gop: do we need to check ConOut any more?

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Date:   Fri, 13 Dec 2019 10:06:32 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Eugene Cohen <eugene@...com>,
        Matthew Garrett <mjg59@...gle.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: efi/gop: do we need to check ConOut any more?

On Thu, 12 Dec 2019 at 23:23, Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> Since commit 38cb5ef4473c ("X86: Improve GOP detection in the EFI boot
> stub") we check for a GOP device that implements ConOut protocol to find
> our primary output device. The commit log says that this was done to
> avoid problems with the ConSplitter device, which exports a virtual GOP
> interface. The original version by Matt Fleming checked for PCIIO
> protocol, with a note that says there are some Apple machines have GOPs
> without hardware, I assume that that was the same case, GOPs from
> ConSplitter.
>
> However, since commit 540f4c0e894f ("efi/libstub: Skip GOP with
> PIXEL_BLT_ONLY format") we skip GOP's that don't have a framebuffer.
>
> Looking at the EDK2 implementation of ConSplitter, the virtual GOP will
> advertise a framebuffer iff it is attached to exactly one GOP device, in
> which case it passes through all the information. If it is attached to a
> UGA device or to more than one GOP, it will show as a non-framebuffer
> GOP, so we will skip it anyway in those cases.
>
> Given that, is it still necessary to check for conout at all, or would
> it be enough to rely on the framebuffer check?
>

If it doesn't hurt to check, I'd prefer to keep it in. UEFI is a can
of worms, given how many buggy implementations exist in the field.