From: Stephen C. Tweedie <sct@redhat.com>
Subject: [RHEL5 patch 1/2] Fix xen swiotlb for b44 module --- kernel patch
Date: Thu, 07 Dec 2006 13:09:37 +0000
Bugzilla: 216472
Message-Id: <1165496977.6309.18.camel@sisko.scot.redhat.com>
Changelog: xen: Fix xen swiotlb for b44 module


Hi,

In conjunction with the following patch (xen hypervisor patch for same
bug), fixes blocker bug:

	Bugzilla Bug 216472: b44 module doesn't work with xen kernel
	https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=216472

The b44 module will not load under xen, failing with the error

        b44 0000:03:00.0: No usable DMA configuration, aborting.
        ACPI: PCI interrupt for device 0000:03:00.0 disabled
        b44: probe of 0000:03:00.0 failed with error -5
        
The trouble is that b44 requests a pci dma mask of 0x3fffffff (ie. 30
usable address lines).  Xen has basic support for this sort of thing,
via a swiotlb in the kernel which is allocated pages from a hypervisor
pool that is restricted to the bottom 2^N bits of physical memory.  But
unfortunately, in RHEL-5, it's only using N=31, not N=30, so it can't
guarantee to give the b44 device pages satisfying that hardware
restriction.

The fix is very simple --- change the swiotlb to request pages
constrained to 2^30 instead of 2^31 (ie. below 1GB instead of 2GB max
physical address), and correspondingly change the hypervisor's DMADOM
memory pool to be constrained to the same limit.  

Patch has been tested and verified by multiple users on FC-6 kernels in
bug 208242, and was merged upstream on Nov. 28.

Please ACK.

--Stephen

commit ae88790a072231867c8104674190a52cbc2f2867
Author: Stephen Tweedie <sct@redhat.com>

    Use 30 bits (1GB) swiotlb physical mapping limit, for b44

diff --git a/arch/i386/kernel/swiotlb.c b/arch/i386/kernel/swiotlb.c
index 7ae10ad..93a4d6c 100644
--- a/arch/i386/kernel/swiotlb.c
+++ b/arch/i386/kernel/swiotlb.c
@@ -47,8 +47,8 @@ #define IO_TLB_SEGSIZE	128
  */
 #define IO_TLB_SHIFT 11
 
-/* Width of DMA addresses in the IO TLB. 31 bits is an aacraid limitation. */
-#define IO_TLB_DMA_BITS 31
+/* Width of DMA addresses in the IO TLB. 30 bits is a b44 limitation. */
+#define IO_TLB_DMA_BITS 30
 
 int swiotlb_force;
 static char *iotlb_virt_start;