linux-kernel - Re: [PATCH 18 of 38] x86: unify pci iommu setup and allow swiotlb to compile for 32 bit

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Message-Id: <1226938566.18916.80.camel@zakaz.uk.xensource.com>
Date:	Mon, 17 Nov 2008 16:16:06 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	jeremy@...p.org, mingo@...e.hu, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com, x86@...nel.org
Subject: Re: [PATCH 18 of 38] x86: unify pci iommu setup and allow swiotlb
	to	compile for 32 bit

On Mon, 2008-11-17 at 12:48 +0900, FUJITA Tomonori wrote:
> On Thu, 13 Nov 2008 11:10:16 -0800
> Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 
> > swiotlb on 32 bit will be used by Xen domain 0 support.
> 
> If you want swiotlb on 32 bit, you need more modifications, I think.

Possibly. It currently "Works For Me(tm)", but I should check it over.

> For example, the following code assumes that the mask needs to be
> 64 bits.

The use of unsigned long for the mask is throughout the API and not
simply limited to swiotlb.c. All the callers of dma_set_seg_boundary
(PCI and SCSI subsys it seems) do not use a value >4G anywhere I can
see. Presumably if something was we would see "warning: overflow in
implicit constant conversion" somewhere along the line. If no value is
set then the default is 0xffffffff which is safe on 32 bit.

I suspect that even with PAE addresses above 4G aren't seen very often
due to pre-existing subsystem specific bounce buffers or other existing
limitations (like network buffers being in lowmem).

Perhaps dma_addr_t should be used though?

Ian.

> 
> static void *
> map_single(struct device *hwdev, char *buffer, size_t size, int dir)
> {
> 	unsigned long flags;
> 	char *dma_addr;
> 	unsigned int nslots, stride, index, wrap;
> 	int i;
> 	unsigned long start_dma_addr;
> 	unsigned long mask;
> 	unsigned long offset_slots;
> 	unsigned long max_slots;
> 
> 	mask = dma_get_seg_boundary(hwdev);
> 	start_dma_addr = virt_to_bus(io_tlb_start) & mask;
> 
> 	offset_slots = ALIGN(start_dma_addr, 1 << IO_TLB_SHIFT) >> IO_TLB_SHIFT;
> 	max_slots = mask + 1
> 		    ? ALIGN(mask + 1, 1 << IO_TLB_SHIFT) >> IO_TLB_SHIFT
> 		    : 1UL << (BITS_PER_LONG - IO_TLB_SHIFT);
> 

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