[PATCH v3] dma: Fix max PFN arithmetic overflow on 32 bit systems
The intermediate result of the old term (4UL * 1024 * 1024 * 1024) is
4 294 967 296 or 0x1 which is no problem on 64 bit systems. The
patch does not change the later overall result of 0x10 for
MAX_DMA32_PFN. The new calculation yields the same result, but does not
require 64 bit arithmetic.
On 32 bit systems the old calculation suffers from an arithmetic
overflow in that intermediate term in braces: 4UL aka unsigned long int
is 4 byte wide and an arithmetic overflow happens (the 0x1 does
not fit in 4 bytes), the in braces result is truncated to zero, the
following right shift does not alter that, so MAX_DMA32_PFN evaluates to
0 on 32 bit systems.
That wrong value is a problem in a comparision against MAX_DMA32_PFN in
the init code for swiotlb in 'pci_swiotlb_detect_4gb()' to decide if
swiotlb should be active. That comparison yields the opposite result,
when compiling on 32 bit systems.
This was not possible before 1b7e03ef7570 ("x86, NUMA: Enable emulation
on 32bit too") when that MAX_DMA32_PFN was first made visible to x86_32
(and which landed in v3.0).
In practice this wasn't a problem, unless you activated CONFIG_SWIOTLB
on x86 (32 bit).
However for ARCH=x86 (32 bit) and if you have 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. That landed in v5.4, where we noticed it in
the fli4l Linux distribution. We have CONFIG_IOMMU_INTEL active on both
32 and 64 bit kernel configs there (I could not find out why, so let's
just say historical reasons).
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 like pcengines ALIX 2D3 with 256 MiB memory, which are still
frequently used as home routers.
We noticed this effect when migrating from kernel v4.19 (LTS) to v5.4
(LTS) in fli4l and got that kernel messages 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
…
Memory: 183484K/261756K available (4594K kernel code, 393K rwdata, 1660K
rodata, 536K init, 456K bss , 78272K reserved, 0K cma-reserved, 0K highmem)
…
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 new calculation, which does not suffer from that overflow, is the
same as for arch/mips now as suggested by Robin Murphy.
The fix was tested by fli4l users on round about two dozen different
systems, including both 32 and 64 bit archs, bare metal and virtualized
machines.
Fixes: 1b7e03ef7570 ("x86, NUMA: Enable emulation on 32bit too")
Fixes: https://web.nettworks.org/bugs/browse/FFL-2560
Fixes: https://unix.stackexchange.com/q/520065/50007
Reported-by: Alan Jenkins
Suggested-by: Robin Murphy
Signed-off-by: Alexander Dahl
Cc: sta...@vger.kernel.org
---
Notes:
v3:
- rewritten commit message to better explain that arithmetic overflow
and added Fixes tag (Greg Kroah-Hartman)
- rebased on v5.7-rc7
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
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Re: [PATCH v2] dma: Fix max PFN arithmetic overflow on 32 bit systems
Hei hei,
I would like to kindly ask about the status of this patch.
On Wed, Apr 15, 2020 at 04:35:21PM +0200, Alexander Dahl wrote:
> now after v5.7-rc1 is out, I would kindly ask, if anyone had time to
> review this one line patch? Is anything wrong with that fix?
Did it maybe not reach the right maintainers? I used
scripts/get_maintainer.pl to get my recipient list.
> (I added the current fli4l kernel package maintainer Florian to Cc to
> let him know I'm still having an eye on this.)
>
> Greets
> Alex
>
> On Sat, Mar 21, 2020 at 07:28:23PM +0100, Alexander Dahl wrote:
> > 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
> > …
> > Memory: 183484K/261756K available (4594K kernel code, 393K rwdata, 1660K
> > rodata, 536K init, 456K bss , 78272K reserved, 0K cma-reserved, 0K highmem)
> > …
> > 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
> > Suggested-by: Robin Murphy
> > Signed-off-by: Alexander Dahl
> > ---
> >
> > 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
Greets
Alex
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Re: [PATCH v2] dma: Fix max PFN arithmetic overflow on 32 bit systems
Hello,
now after v5.7-rc1 is out, I would kindly ask, if anyone had time to
review this one line patch? Is anything wrong with that fix?
(I added the current fli4l kernel package maintainer Florian to Cc to
let him know I'm still having an eye on this.)
Greets
Alex
On Sat, Mar 21, 2020 at 07:28:23PM +0100, Alexander Dahl wrote:
> 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
> …
> Memory: 183484K/261756K available (4594K kernel code, 393K rwdata, 1660K
> rodata, 536K init, 456K bss , 78272K reserved, 0K cma-reserved, 0K highmem)
> …
> 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
> Suggested-by: Robin Murphy
> Signed-off-by: Alexander Dahl
> ---
>
> 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
>
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[PATCH v2] dma: Fix max PFN arithmetic overflow on 32 bit systems
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
…
Memory: 183484K/261756K available (4594K kernel code, 393K rwdata, 1660K
rodata, 536K init, 456K bss , 78272K reserved, 0K cma-reserved, 0K highmem)
…
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
Suggested-by: Robin Murphy
Signed-off-by: Alexander Dahl
---
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
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Re: [PATCH] dma: Fix max PFN arithmetic overflow on 32 bit systems
Hello Robin, On Thu, Mar 19, 2020 at 01:50:56PM +, Robin Murphy wrote: > On 2020-03-02 6:16 pm, Alexander Dahl wrote: > > --- > > 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..e25514eca8d6 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 (4UL * ((1024 * 1024 * 1024) >> PAGE_SHIFT)) > > FWIW, wouldn't s/UL/ULL/ in the original expression suffice? Failing that, > rather than awkward parenthesis trickery it might be clearer to just copy > the one from arch/mips/include/asm/dma.h. Both of your suggestions yield the correct result, and at least for me both look easier to understand than what Alan proposed in the first place. I would opt for the variant which is already in arch/mips, because it avoids 64 bit calculation, is most obvious in intent in my eyes, and we have the same calculation twice then instead of two variants. Thanks for your review, I'll send a v2. :-) Greets Alex -- /"\ ASCII RIBBON | »With the first link, the chain is forged. The first \ / CAMPAIGN | speech censured, the first thought forbidden, the X AGAINST | first freedom denied, chains us all irrevocably.« / \ HTML MAIL| (Jean-Luc Picard, quoting Judge Aaron Satie) signature.asc Description: PGP signature ___ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
Re: [PATCH] dma: Fix max PFN arithmetic overflow on 32 bit systems
Hei hei,
gentle ping on this patch from two weeks ago. Did anyone have time to
look into it? Did I miss someone in Cc or sent it to the wrong lists
maybe?
On Mon, Mar 02, 2020 at 07:16:12PM +0100, Alexander Dahl wrote:
> 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
> …
> Memory: 183484K/261756K available (4594K kernel code, 393K rwdata, 1660K
> rodata, 536K init, 456K bss , 78272K reserved, 0K cma-reserved, 0K highmem)
> …
> 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
>
> Fixes: https://web.nettworks.org/bugs/browse/FFL-2560
> Fixes: https://unix.stackexchange.com/q/520065/50007
> Suggested-by: Alan Jenkins
> Signed-off-by: Alexander Dahl
> ---
> We tested this in qemu and on real hardware with fli4l on top of v5.4,
> v5.5, and v5.6-rc kernels, but only as far as the reserved memory goes.
> The patch itself is based on v5.6-rc3 (IIRC).
We had no complaints of our fli4l users, since we applied this patch
to our distribution kernels.
Thanks & greets
Alex
>
> A quick grep over the kernel code showed me this define MAX_DMA32_PFN is
> used in other places as well. I would appreciate feedback on this,
> because I can not oversee all side effects this might have?!
>
> Thanks again to Alan who proposed the fix, and for his permission to
> send it upstream.
>
> Greets
> Alex
> ---
> 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..e25514eca8d6 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 (4UL * ((1024 * 1024 * 1024) >> PAGE_SHIFT))
>
> #ifdef CONFIG_X86_32
> /* The maximum address that we can perform a DMA transfer to on this
> platform */
> --
> 2.20.1
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[PATCH] dma: Fix max PFN arithmetic overflow on 32 bit systems
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
…
Memory: 183484K/261756K available (4594K kernel code, 393K rwdata, 1660K
rodata, 536K init, 456K bss , 78272K reserved, 0K cma-reserved, 0K highmem)
…
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
Fixes: https://web.nettworks.org/bugs/browse/FFL-2560
Fixes: https://unix.stackexchange.com/q/520065/50007
Suggested-by: Alan Jenkins
Signed-off-by: Alexander Dahl
---
We tested this in qemu and on real hardware with fli4l on top of v5.4,
v5.5, and v5.6-rc kernels, but only as far as the reserved memory goes.
The patch itself is based on v5.6-rc3 (IIRC).
A quick grep over the kernel code showed me this define MAX_DMA32_PFN is
used in other places as well. I would appreciate feedback on this,
because I can not oversee all side effects this might have?!
Thanks again to Alan who proposed the fix, and for his permission to
send it upstream.
Greets
Alex
---
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..e25514eca8d6 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 (4UL * ((1024 * 1024 * 1024) >> PAGE_SHIFT))
#ifdef CONFIG_X86_32
/* The maximum address that we can perform a DMA transfer to on this platform
*/
--
2.20.1
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