[PATCH v2] dma: Fix max PFN arithmetic overflow on 32 bit systems

[Alexander Dahl]

For ARCH=x86 (32 bit) when you set CONFIG_IOMMU_INTEL since c5a5dc4cbbf4
("iommu/vt-d: Don't switch off swiotlb if bounce page is used") there's
a dependency on CONFIG_SWIOTLB, which was not necessarily active before.

The init code for swiotlb in 'pci_swiotlb_detect_4gb()' compares
something against MAX_DMA32_PFN to decide if it should be active.
However that define suffers from an arithmetic overflow since
1b7e03ef7570 ("x86, NUMA: Enable emulation on 32bit too") when it was
first made visible to x86_32.

The effect is at boot time 64 MiB (default size) were allocated for
bounce buffers now, which is a noticeable amount of memory on small
systems. We noticed this effect on the fli4l Linux distribution when
migrating from kernel v4.19 (LTS) to v5.4 (LTS) on boards like pcengines
ALIX 2D3 with 256 MiB memory for example:

  Linux version 5.4.22 (buildroot@buildroot) (gcc version 7.3.0 (Buildroot 2018.02.8)) #1 SMP Mon Nov 26 23:40:00 CET 2018
  â
  Memory: 183484K/261756K available (4594K kernel code, 393K rwdata, 1660K rodata, 536K init, 456K bss , 78272K reserved, 0K cma-reserved, 0K highmem)
  â
  PCI-DMA: Using software bounce buffering for IO (SWIOTLB)
  software IO TLB: mapped [mem 0x0bb78000-0x0fb78000] (64MB)

The initial analysis and the suggested fix was done by user 'sourcejedi'
at stackoverflow and explicitly marked as GPLv2 for inclusion in the
Linux kernel:

  https://unix.stackexchange.com/a/520525/50007

The actual calculation however is the same as for arch/mips now as
suggested by Robin Murphy.

Fixes: https://web.nettworks.org/bugs/browse/FFL-2560
Fixes: https://unix.stackexchange.com/q/520065/50007
Reported-by: Alan Jenkins <alan.christopher.jenkins@xxxxxxxxx>
Suggested-by: Robin Murphy <robin.murphy@xxxxxxx>
Signed-off-by: Alexander Dahl <post@xxxxxxxxxxx>
---

Notes:
    v1 -> v2:
      - use the same calculation as with arch/mips (Robin Murphy)

 arch/x86/include/asm/dma.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/dma.h b/arch/x86/include/asm/dma.h
index 00f7cf45e699..8e95aa4b0d17 100644
--- a/arch/x86/include/asm/dma.h
+++ b/arch/x86/include/asm/dma.h
@@ -74,7 +74,7 @@
 #define MAX_DMA_PFN   ((16UL * 1024 * 1024) >> PAGE_SHIFT)
 
 /* 4GB broken PCI/AGP hardware bus master zone */
-#define MAX_DMA32_PFN ((4UL * 1024 * 1024 * 1024) >> PAGE_SHIFT)
+#define MAX_DMA32_PFN (1UL << (32 - PAGE_SHIFT))
 
 #ifdef CONFIG_X86_32
 /* The maximum address that we can perform a DMA transfer to on this platform */
-- 
2.20.1