Re: [PATCH] ARM: convert max_pfn and max_low_pfn to be relative to PFN0

[Colin Cross]

On Thu, Jun 13, 2013 at 7:37 AM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> On Wed, Jun 12, 2013 at 07:13:23PM -0700, Colin Cross wrote:
>> >From code inspection, I believe this will also improve block device
>> performance where the bounce limit was set to BLK_BOUNCE_HIGH, which
>> was bouncing unnecessarily for the top PHYS_PFN_OFFSET pages of low
>> memory.
>
> This has the potential to break platforms.  The problem is the duality
> of the dma_mask - is it a mask of the bits which the device can drive,
> or a PFN limit.  The block layer interprets it as a PFN limit, because
> of course everywhere starts their memory at physical address zero.

I've never come across a device with dma_mask set to anything but
0xFFFFFFFF, and a quick search didn't find me any good examples.
dma_mask set to the mask of bits the device can drive seems logical,
and it doesn't seem hard to fix the block layer (and the few other
users of max_pfn) to use min_low_pfn << PAGE_SHIFT + dma_mask.

> This gets into a world of pain if you have any of these conditions:
> (a) RAM not starting at physical address zero
The device I tested on has RAM at 0x40000000, which is what caused my problem.

> (b) Any translation between physical addresses and bus addresses
Is that just footbridge, integrator, and ks8695?  Those are the only
machines that define __virt_to_bus

> What we know is that the existing stuff works.  What we don't know is
> whether changing it will break anything which falls into the above
> two categories.

The existing stuff breaks a userspace API, /proc/kpagecount and
/proc/kpageflags.  It is currently impossible to read the page at
max_pfn on ARM.  It is possible to read the pages after max_pfn
because of an underflow in the kpagecount bounds check, which doesn't
cause problems because it checks pfn_valid on every pfn.  Just taking
out the bounds check against max_pfn in kpagecount and kpageflags
would also fix my problem, but it seems correct on everything but ARM.
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