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Date: Thu, 13 Jun 2013 11:30:20 -0700 From: Colin Cross <ccross@...roid.com> To: Russell King - ARM Linux <linux@....linux.org.uk> Cc: "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, open list <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] ARM: convert max_pfn and max_low_pfn to be relative to PFN0 On Thu, Jun 13, 2013 at 7:37 AM, Russell King - ARM Linux <linux@....linux.org.uk> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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