linux-kernel - Re: [PATCH 6/6] efi/earlycon: Remap entire framebuffer after page initialization

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Message-ID: <20191210200546.GA55356@rani.riverdale.lan>
Date:   Tue, 10 Dec 2019 15:05:46 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>
Subject: Re: [PATCH 6/6] efi/earlycon: Remap entire framebuffer after page
 initialization

On Mon, Dec 09, 2019 at 07:24:13PM +0000, Ard Biesheuvel wrote:
> On Mon, 9 Dec 2019 at 20:12, Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > On Fri, Dec 06, 2019 at 04:55:42PM +0000, Ard Biesheuvel wrote:
> > > From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > >
> > > When commit 69c1f396f25b
> > >
> > >   "efi/x86: Convert x86 EFI earlyprintk into generic earlycon implementation"
> > >
> > > moved the x86 specific EFI earlyprintk implementation to a shared location,
> > > it also tweaked the behaviour. In particular, it dropped a trick with full
> > > framebuffer remapping after page initialization, leading to two regressions:
> > > 1) very slow scrolling after page initialization,
> > > 2) kernel hang when the 'keep_bootcon' command line argument is passed.
> > >
> > > Putting the tweak back fixes #2 and mitigates #1, i.e., it limits the slow
> > > behavior to the early boot stages, presumably due to eliminating heavy
> > > map()/unmap() operations per each pixel line on the screen.
> > >
> >
> > Could the efi earlycon have an interaction with PCI resource allocation,
> > similar to what commit dcf8f5ce3165 ("drivers/fbdev/efifb: Allow BAR to
> > be moved instead of claiming it") fixed for efifb?
> 
> Yes. If the BAR gets moved, things will break. This is mostly an issue
> for the keep_bootcon case, but that is documented as being a debug
> feature specifically for addressing console initialization related
> issues. Earlycon itself is also a debug feature, so if you hit the BAR
> reallocation issue, you're simply out of luck. Note that this happens
> rarely in practice, only on non-x86 systems where the firmware and the
> kernel have very different policies regarding BAR allocation, and on
> DT based systems, you can force the OS to honour the existing
> allocation by using linux,pci-probe-only

Thanks. Another q -- I tried out the earlycon=efifb, and it seems like
it gets disabled (without keep_bootcon) as soon as dummycon takes over,
which is well before the real console.

DUMMY_CONSOLE is defined as
	depends on VGA_CONSOLE!=y || SGI_NEWPORT_CONSOLE!=y 
	default y

so it seems like it will pretty much always be enabled, as it doesn't
seem likely that VGA_CONSOLE=y and SGI_NEWPORT_CONSOLE=y would ever be
true simultaneously.

Am I missing something or is this the way it's supposed to work? So
keep_bootcon seems almost necessary with the EFI boot console? Would a
patch to not disable boot console when dummycon is initialized, but wait
for a real console, be useful?

Thanks.