linux-kernel - Re: [PATCH] arm64: fixmap: fix missing sub-page offset for earlyprintk

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Date:	Mon, 28 Apr 2014 18:51:18 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Mark Salter <msalter@...hat.com>
Subject: Re: [PATCH] arm64: fixmap: fix missing sub-page offset for earlyprintk

On Mon, Apr 28, 2014 at 1:50 PM, Marc Zyngier <marc.zyngier@....com> wrote:
> Commit d57c33c5daa4 (add generic fixmap.h) added (among other
> similar things) set_fixmap_io to deal with early ioremap of devices.
>
> More recently, commit bf4b558eba92 (arm64: add early_ioremap support)
> converted the arm64 earlyprintk to use set_fixmap_io. A side effect of
> this conversion is that my virtual machines have stopped booting when
> I pass "earlyprintk=uart8250-8bit,0x3f8" to the guest kernel.
>
> Turns out that the new earlyprintk code doesn't care at all about
> sub-page offsets, and just assumes that the earlyprintk device will
> be page-aligned. Obviously, that doesn't play well with the above example.
>
> Further investigation shows that set_fixmap_io uses __set_fixmap instead
> of __set_fixmap_offset. A fix is to introduce a set_fixmap_offset_io that
> uses the latter, and to remove the superflous call to fix_to_virt
> (which only returns the value that set_fixmap_io has already given us).
>
> With this applied, my VMs are back in business. Tested on a Cortex-A57
> platform with kvmtool as platform emulation.
>
> Cc: Mark Salter <msalter@...hat.com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
> ---
>  arch/arm64/kernel/early_printk.c | 6 ++----
>  include/asm-generic/fixmap.h     | 3 +++
>  2 files changed, 5 insertions(+), 4 deletions(-)

This will be fixed already in 3.16 with my earlycon series[1] if this
is not taken for 3.15.

Probably still a good addition for fixmap.

Rob

[1] https://lkml.org/lkml/2014/4/18/573
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