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

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Message-ID: <20191211155503.GA204038@rani.riverdale.lan>
Date:   Wed, 11 Dec 2019 10:55:03 -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 Wed, Dec 11, 2019 at 11:26:17AM +0000, Ard Biesheuvel wrote:
> On Tue, 10 Dec 2019 at 21:05, Arvind Sankar <nivedita@...m.mit.edu> wrote:
> > 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?
> >
> 
> Well spotted!
> 
> I have traced this down to [0] which combined various arch specific
> definitions into one, and obviously chose the wrong boolean operator
> for combining the conditions.
> 
> Patches welcome.
> 
> 
> [0] https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/drivers/video/console/Kconfig?id=31d2a7d36d6989c714b792ec00358ada24c039e7

Thanks! I was a little mistaken about what actually disables it though,
it seems to be when vt_console_driver gets registered, and at that point
the vt is still using dummy_con. I had thought dummy_con itself was a
console driver.