linux-kernel - Re: Regression (ARM) arch/arm/mm/init.c doesn't build without CONFIG_ARM_PATCH_PHYS

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Message-ID: <20140106223906.GH27432@n2100.arm.linux.org.uk>
Date:	Mon, 6 Jan 2014 22:39:06 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Krzysztof Hałasa <khalasa@...p.pl>
Cc:	Santosh Shilimkar <santosh.shilimkar@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Regression (ARM) arch/arm/mm/init.c doesn't build without
	CONFIG_ARM_PATCH_PHYS_VIRT.

On Mon, Jan 06, 2014 at 08:28:20PM +0100, Krzysztof Hałasa wrote:
> Russell, Santosh,
> 
> the unneeded commit causing regression is still in place. Please try to
> compile an ARM kernel without CONFIG_ARM_PATCH_PHYS_VIRT and with
> CONFIG_ZONE_DMA and see for yourself, if you don't believe me.
> 
> Please be aware that this commit fixes nothing, its only function is
> causing the regression - so we don't lose anything by reverting it.
> 
> If the attached wasn't clear, what the defective commit presently does
> is changing a perfectly valid code into a code referencing a variable
> which (without CONFIG_ARM_PATCH_PHYS_VIRT set) doesn't at all exist.
> 
> With CONFIG_ARM_PATCH_PHYS_VIRT set, this commit does precisely nothing.

Right, so, with Assabet, which has CONFIG_DMA_ZONE=y and
CONFIG_ARM_PATCH_PHYS_VIRT=y:

$ make O=../build/assabet arch/arm/mm/init.i

gives:
  arm_dma_limit = __pv_phys_offset + arm_dma_zone_size - 1;

with or without Santosh's patch.  So, with V2P patching in place, there's
absolutely no functional difference.

With CONFIG_ARM_PATCH_PHYS_VIRT=n, before Santosh's patch:

  arm_dma_limit = (0xc0000000UL) + arm_dma_zone_size - 1;

After:

  arm_dma_limit = __pv_phys_offset + arm_dma_zone_size - 1;

and this breaks the build because there is no __pv_phys_offset symbol.

Now, the case which matters for Santosh is the first case - the one
where CONFIG_ARM_PATCH_PHYS_VIRT=y.  We can clearly see that after
preprocessing, the results are 100% identical.

Therefore, I find myself agreeing with Krzysztof that the commit is
bad, has no functional change for the case it was proposed to solve,
and needs to be reverted.

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