Re: [RFC/RFT PATCH 4/5] ARM: mm: change max*pfn to include the physical offset of memory

2013-07-31 Thread Santosh Shilimkar 
On Wednesday 31 July 2013 06:56 AM, Russell King - ARM Linux wrote:
> On Fri, Jul 12, 2013 at 05:48:13PM -0400, Santosh Shilimkar wrote:
>> diff --git a/arch/arm/include/asm/dma-mapping.h 
>> b/arch/arm/include/asm/dma-mapping.h
>> index 5b579b9..b2d5937 100644
>> --- a/arch/arm/include/asm/dma-mapping.h
>> +++ b/arch/arm/include/asm/dma-mapping.h
>> @@ -47,12 +47,12 @@ static inline int dma_set_mask(struct device *dev, u64 
>> mask)
>>  #ifndef __arch_pfn_to_dma
>>  static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
>>  {
>> -return (dma_addr_t)__pfn_to_bus(pfn);
>> +return (dma_addr_t)__pfn_to_bus(pfn + PHYS_PFN_OFFSET);
>>  }
>>  
>>  static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
>>  {
>> -return __bus_to_pfn(addr);
>> +return __bus_to_pfn(addr) - PHYS_PFN_OFFSET;
>>  }
>>  
>>  static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
>> @@ -64,15 +64,16 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
>> void *addr)
>>  {
>>  return (dma_addr_t)__virt_to_bus((unsigned long)(addr));
>>  }
>> +
>>  #else
>>  static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
>>  {
>> -return __arch_pfn_to_dma(dev, pfn);
>> +return __arch_pfn_to_dma(dev, pfn + PHYS_PFN_OFFSET);
>>  }
>>  
>>  static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
>>  {
>> -return __arch_dma_to_pfn(dev, addr);
>> +return __arch_dma_to_pfn(dev, addr) - PHYS_PFN_OFFSET;
>>  }
>>  
>>  static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
>> @@ -86,6 +87,13 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
>> void *addr)
>>  }
>>  #endif
> 
> Note that I don't think the above are correct - the 'pfn' argument to the
> above functions already includes the PFN offset of physical memory -
> they're all physical_address >> PAGE_SHIFT values.
> 
Right.
Updated patch pushed into the patch system. (patch 7805/1)

Regards,
Santosh
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Re: [RFC/RFT PATCH 4/5] ARM: mm: change max*pfn to include the physical offset of memory

2013-07-31 Thread Russell King - ARM Linux 
On Fri, Jul 12, 2013 at 05:48:13PM -0400, Santosh Shilimkar wrote:
> diff --git a/arch/arm/include/asm/dma-mapping.h 
> b/arch/arm/include/asm/dma-mapping.h
> index 5b579b9..b2d5937 100644
> --- a/arch/arm/include/asm/dma-mapping.h
> +++ b/arch/arm/include/asm/dma-mapping.h
> @@ -47,12 +47,12 @@ static inline int dma_set_mask(struct device *dev, u64 
> mask)
>  #ifndef __arch_pfn_to_dma
>  static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
>  {
> - return (dma_addr_t)__pfn_to_bus(pfn);
> + return (dma_addr_t)__pfn_to_bus(pfn + PHYS_PFN_OFFSET);
>  }
>  
>  static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
>  {
> - return __bus_to_pfn(addr);
> + return __bus_to_pfn(addr) - PHYS_PFN_OFFSET;
>  }
>  
>  static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
> @@ -64,15 +64,16 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
> void *addr)
>  {
>   return (dma_addr_t)__virt_to_bus((unsigned long)(addr));
>  }
> +
>  #else
>  static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
>  {
> - return __arch_pfn_to_dma(dev, pfn);
> + return __arch_pfn_to_dma(dev, pfn + PHYS_PFN_OFFSET);
>  }
>  
>  static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
>  {
> - return __arch_dma_to_pfn(dev, addr);
> + return __arch_dma_to_pfn(dev, addr) - PHYS_PFN_OFFSET;
>  }
>  
>  static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
> @@ -86,6 +87,13 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
> void *addr)
>  }
>  #endif

Note that I don't think the above are correct - the 'pfn' argument to the
above functions already includes the PFN offset of physical memory -
they're all physical_address >> PAGE_SHIFT values.

> +/* The ARM override for dma_max_pfn() */
> +static inline unsigned long dma_max_pfn(struct device *dev)
> +{
> + return dma_to_pfn(dev, *dev->dma_mask);
> +}
> +#define dma_max_pfn(dev) dma_max_pfn(dev)

So I'd suggest this becomes:

return PHYS_PFN_OFFSET + dma_to_pfn(dev, *dev->dma_mask);
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Re: [RFC/RFT PATCH 4/5] ARM: mm: change max*pfn to include the physical offset of memory

2013-07-31 Thread Russell King - ARM Linux 
On Fri, Jul 12, 2013 at 05:48:13PM -0400, Santosh Shilimkar wrote:
 diff --git a/arch/arm/include/asm/dma-mapping.h 
 b/arch/arm/include/asm/dma-mapping.h
 index 5b579b9..b2d5937 100644
 --- a/arch/arm/include/asm/dma-mapping.h
 +++ b/arch/arm/include/asm/dma-mapping.h
 @@ -47,12 +47,12 @@ static inline int dma_set_mask(struct device *dev, u64 
 mask)
  #ifndef __arch_pfn_to_dma
  static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
  {
 - return (dma_addr_t)__pfn_to_bus(pfn);
 + return (dma_addr_t)__pfn_to_bus(pfn + PHYS_PFN_OFFSET);
  }
  
  static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
  {
 - return __bus_to_pfn(addr);
 + return __bus_to_pfn(addr) - PHYS_PFN_OFFSET;
  }
  
  static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
 @@ -64,15 +64,16 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
 void *addr)
  {
   return (dma_addr_t)__virt_to_bus((unsigned long)(addr));
  }
 +
  #else
  static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
  {
 - return __arch_pfn_to_dma(dev, pfn);
 + return __arch_pfn_to_dma(dev, pfn + PHYS_PFN_OFFSET);
  }
  
  static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
  {
 - return __arch_dma_to_pfn(dev, addr);
 + return __arch_dma_to_pfn(dev, addr) - PHYS_PFN_OFFSET;
  }
  
  static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
 @@ -86,6 +87,13 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
 void *addr)
  }
  #endif

Note that I don't think the above are correct - the 'pfn' argument to the
above functions already includes the PFN offset of physical memory -
they're all physical_address  PAGE_SHIFT values.

 +/* The ARM override for dma_max_pfn() */
 +static inline unsigned long dma_max_pfn(struct device *dev)
 +{
 + return dma_to_pfn(dev, *dev-dma_mask);
 +}
 +#define dma_max_pfn(dev) dma_max_pfn(dev)

So I'd suggest this becomes:

return PHYS_PFN_OFFSET + dma_to_pfn(dev, *dev-dma_mask);
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Re: [RFC/RFT PATCH 4/5] ARM: mm: change max*pfn to include the physical offset of memory

2013-07-31 Thread Santosh Shilimkar 
On Wednesday 31 July 2013 06:56 AM, Russell King - ARM Linux wrote:
 On Fri, Jul 12, 2013 at 05:48:13PM -0400, Santosh Shilimkar wrote:
 diff --git a/arch/arm/include/asm/dma-mapping.h 
 b/arch/arm/include/asm/dma-mapping.h
 index 5b579b9..b2d5937 100644
 --- a/arch/arm/include/asm/dma-mapping.h
 +++ b/arch/arm/include/asm/dma-mapping.h
 @@ -47,12 +47,12 @@ static inline int dma_set_mask(struct device *dev, u64 
 mask)
  #ifndef __arch_pfn_to_dma
  static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
  {
 -return (dma_addr_t)__pfn_to_bus(pfn);
 +return (dma_addr_t)__pfn_to_bus(pfn + PHYS_PFN_OFFSET);
  }
  
  static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
  {
 -return __bus_to_pfn(addr);
 +return __bus_to_pfn(addr) - PHYS_PFN_OFFSET;
  }
  
  static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
 @@ -64,15 +64,16 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
 void *addr)
  {
  return (dma_addr_t)__virt_to_bus((unsigned long)(addr));
  }
 +
  #else
  static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
  {
 -return __arch_pfn_to_dma(dev, pfn);
 +return __arch_pfn_to_dma(dev, pfn + PHYS_PFN_OFFSET);
  }
  
  static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
  {
 -return __arch_dma_to_pfn(dev, addr);
 +return __arch_dma_to_pfn(dev, addr) - PHYS_PFN_OFFSET;
  }
  
  static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
 @@ -86,6 +87,13 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
 void *addr)
  }
  #endif
 
 Note that I don't think the above are correct - the 'pfn' argument to the
 above functions already includes the PFN offset of physical memory -
 they're all physical_address  PAGE_SHIFT values.
 
Right.
Updated patch pushed into the patch system. (patch 7805/1)

Regards,
Santosh
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Re: [RFC/RFT PATCH 4/5] ARM: mm: change max*pfn to include the physical offset of memory

2013-07-12 Thread Russell King - ARM Linux 
On Fri, Jul 12, 2013 at 05:48:13PM -0400, Santosh Shilimkar wrote:
> Most of the kernel code assumes that max*pfn is maximum pfns because
> the physical start of memory is expected to be PFN0. Since this
> assumption is not true on ARM architectures, the meaning of max*pfn
> is number of memory pages. This is done to keep drivers happy which
> are making use of of these variable to calculate the dma bounce limit
> using dma_mask.
> 
> Now since we have a architecture override possibility for DMAable
> maximum pfns, lets make meaning of max*pfns as maximum pnfs on ARM
> as well.
> 
> In the patch, the dma_to_pfn/pfn_to_dma() pair is hacked to take care of
> the physical memory offset. It is done this way just to enable testing
> since its understood that it can come in way of single zImage.

As Santosh says, this is a hack - but we need to have a discussion about
how to handle translations from PFN to bus addresses.

Currently, the way we do that on ARM is mostly assume that physical
addresses are the same as bus addresses, but that's not true everywhere,
and certainly isn't true when you have a 32-bit DMA controller which
can access physical memory, where the physical memory is above 4GB
in physical space.

We have certain platforms where the DMA address is already being
programmed into a controller with less than 32-bits in its address
register, and with a physical memory offset - and of course this
case just works out of the box because the high bits are ignored
by the device.

What I'm basically saying is we've had this problem for a while, and
we've lived with it by hoping and hacking, and adjusting max*pfn, but
this is not long-term sustainable.  We *need* to get away from the
idea that DMA addresses are physical addresses and device DMA masks
have some relationship to physical addresses.

Consider for a moment:

PCI address 0x ---> physical address 0xc000.

You plug a card in which can't do 32-bit addressing (remember, there
are such PCI cards in the past...).  The driver sets the DMA mask to
0x0fff (or whatever).  How does that relate to the PCI bus address?
It's 0x to 0x0fff.  How does that relate to the physical
address space?  0xc000 to 0xcfff.

This is why DMA masks can't be treated as some notional address limit.
It just doesn't work when you have bus offsets.

And the extreme case of that is LPAE with all system memory above the
4GB physical mark, with 32-bit DMA capable peripherals - which we're
starting to see now.

Ideally, I think we need some kind of per-bus DT property to describe
the memory which can be accessed from the bus - to do it properly to
cover the cases we've already seen, that would be an offset and a size.

We then need some way for dma_to_pfn() and pfn_to_dma() to efficiently
get at that information - bear in mind that they're hot paths when doing
DMA mappings and the like.  I doubt we want to be looking up the same
property time and time again inside them.
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[RFC/RFT PATCH 4/5] ARM: mm: change max*pfn to include the physical offset of memory

2013-07-12 Thread Santosh Shilimkar 
Most of the kernel code assumes that max*pfn is maximum pfns because
the physical start of memory is expected to be PFN0. Since this
assumption is not true on ARM architectures, the meaning of max*pfn
is number of memory pages. This is done to keep drivers happy which
are making use of of these variable to calculate the dma bounce limit
using dma_mask.

Now since we have a architecture override possibility for DMAable
maximum pfns, lets make meaning of max*pfns as maximum pnfs on ARM
as well.

In the patch, the dma_to_pfn/pfn_to_dma() pair is hacked to take care of
the physical memory offset. It is done this way just to enable testing
since its understood that it can come in way of single zImage.

Cc: Russell King 
Cc: Catalin Marinas 
Cc: Will Deacon 
Cc: Nicolas Pitre 

Signed-off-by: Santosh Shilimkar 
---
 arch/arm/include/asm/dma-mapping.h |   16 
 arch/arm/mm/init.c |   10 --
 2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/arch/arm/include/asm/dma-mapping.h 
b/arch/arm/include/asm/dma-mapping.h
index 5b579b9..b2d5937 100644
--- a/arch/arm/include/asm/dma-mapping.h
+++ b/arch/arm/include/asm/dma-mapping.h
@@ -47,12 +47,12 @@ static inline int dma_set_mask(struct device *dev, u64 mask)
 #ifndef __arch_pfn_to_dma
 static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
 {
-   return (dma_addr_t)__pfn_to_bus(pfn);
+   return (dma_addr_t)__pfn_to_bus(pfn + PHYS_PFN_OFFSET);
 }
 
 static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
 {
-   return __bus_to_pfn(addr);
+   return __bus_to_pfn(addr) - PHYS_PFN_OFFSET;
 }
 
 static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
@@ -64,15 +64,16 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
void *addr)
 {
return (dma_addr_t)__virt_to_bus((unsigned long)(addr));
 }
+
 #else
 static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
 {
-   return __arch_pfn_to_dma(dev, pfn);
+   return __arch_pfn_to_dma(dev, pfn + PHYS_PFN_OFFSET);
 }
 
 static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
 {
-   return __arch_dma_to_pfn(dev, addr);
+   return __arch_dma_to_pfn(dev, addr) - PHYS_PFN_OFFSET;
 }
 
 static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
@@ -86,6 +87,13 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
void *addr)
 }
 #endif
 
+/* The ARM override for dma_max_pfn() */
+static inline unsigned long dma_max_pfn(struct device *dev)
+{
+   return dma_to_pfn(dev, *dev->dma_mask);
+}
+#define dma_max_pfn(dev) dma_max_pfn(dev)
+
 /*
  * DMA errors are defined by all-bits-set in the DMA address.
  */
diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 6833cbe..588a2c1 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -420,12 +420,10 @@ void __init bootmem_init(void)
 * This doesn't seem to be used by the Linux memory manager any
 * more, but is used by ll_rw_block.  If we can get rid of it, we
 * also get rid of some of the stuff above as well.
-*
-* Note: max_low_pfn and max_pfn reflect the number of _pages_ in
-* the system, not the maximum PFN.
 */
-   max_low_pfn = max_low - PHYS_PFN_OFFSET;
-   max_pfn = max_high - PHYS_PFN_OFFSET;
+   min_low_pfn = min;
+   max_low_pfn = max_low;
+   max_pfn = max_high;
 }
 
 /*
@@ -531,7 +529,7 @@ static inline void free_area_high(unsigned long pfn, 
unsigned long end)
 static void __init free_highpages(void)
 {
 #ifdef CONFIG_HIGHMEM
-   unsigned long max_low = max_low_pfn + PHYS_PFN_OFFSET;
+   unsigned long max_low = max_low_pfn;
struct memblock_region *mem, *res;
 
/* set highmem page free */
-- 
1.7.9.5

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[RFC/RFT PATCH 4/5] ARM: mm: change max*pfn to include the physical offset of memory

2013-07-12 Thread Santosh Shilimkar 
Most of the kernel code assumes that max*pfn is maximum pfns because
the physical start of memory is expected to be PFN0. Since this
assumption is not true on ARM architectures, the meaning of max*pfn
is number of memory pages. This is done to keep drivers happy which
are making use of of these variable to calculate the dma bounce limit
using dma_mask.

Now since we have a architecture override possibility for DMAable
maximum pfns, lets make meaning of max*pfns as maximum pnfs on ARM
as well.

In the patch, the dma_to_pfn/pfn_to_dma() pair is hacked to take care of
the physical memory offset. It is done this way just to enable testing
since its understood that it can come in way of single zImage.

Cc: Russell King li...@arm.linux.org.uk
Cc: Catalin Marinas catalin.mari...@arm.com
Cc: Will Deacon will.dea...@arm.com
Cc: Nicolas Pitre nicolas.pi...@linaro.org

Signed-off-by: Santosh Shilimkar santosh.shilim...@ti.com
---
 arch/arm/include/asm/dma-mapping.h |   16 
 arch/arm/mm/init.c |   10 --
 2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/arch/arm/include/asm/dma-mapping.h 
b/arch/arm/include/asm/dma-mapping.h
index 5b579b9..b2d5937 100644
--- a/arch/arm/include/asm/dma-mapping.h
+++ b/arch/arm/include/asm/dma-mapping.h
@@ -47,12 +47,12 @@ static inline int dma_set_mask(struct device *dev, u64 mask)
 #ifndef __arch_pfn_to_dma
 static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
 {
-   return (dma_addr_t)__pfn_to_bus(pfn);
+   return (dma_addr_t)__pfn_to_bus(pfn + PHYS_PFN_OFFSET);
 }
 
 static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
 {
-   return __bus_to_pfn(addr);
+   return __bus_to_pfn(addr) - PHYS_PFN_OFFSET;
 }
 
 static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
@@ -64,15 +64,16 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
void *addr)
 {
return (dma_addr_t)__virt_to_bus((unsigned long)(addr));
 }
+
 #else
 static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
 {
-   return __arch_pfn_to_dma(dev, pfn);
+   return __arch_pfn_to_dma(dev, pfn + PHYS_PFN_OFFSET);
 }
 
 static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
 {
-   return __arch_dma_to_pfn(dev, addr);
+   return __arch_dma_to_pfn(dev, addr) - PHYS_PFN_OFFSET;
 }
 
 static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
@@ -86,6 +87,13 @@ static inline dma_addr_t virt_to_dma(struct device *dev, 
void *addr)
 }
 #endif
 
+/* The ARM override for dma_max_pfn() */
+static inline unsigned long dma_max_pfn(struct device *dev)
+{
+   return dma_to_pfn(dev, *dev-dma_mask);
+}
+#define dma_max_pfn(dev) dma_max_pfn(dev)
+
 /*
  * DMA errors are defined by all-bits-set in the DMA address.
  */
diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 6833cbe..588a2c1 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -420,12 +420,10 @@ void __init bootmem_init(void)
 * This doesn't seem to be used by the Linux memory manager any
 * more, but is used by ll_rw_block.  If we can get rid of it, we
 * also get rid of some of the stuff above as well.
-*
-* Note: max_low_pfn and max_pfn reflect the number of _pages_ in
-* the system, not the maximum PFN.
 */
-   max_low_pfn = max_low - PHYS_PFN_OFFSET;
-   max_pfn = max_high - PHYS_PFN_OFFSET;
+   min_low_pfn = min;
+   max_low_pfn = max_low;
+   max_pfn = max_high;
 }
 
 /*
@@ -531,7 +529,7 @@ static inline void free_area_high(unsigned long pfn, 
unsigned long end)
 static void __init free_highpages(void)
 {
 #ifdef CONFIG_HIGHMEM
-   unsigned long max_low = max_low_pfn + PHYS_PFN_OFFSET;
+   unsigned long max_low = max_low_pfn;
struct memblock_region *mem, *res;
 
/* set highmem page free */
-- 
1.7.9.5

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Re: [RFC/RFT PATCH 4/5] ARM: mm: change max*pfn to include the physical offset of memory

2013-07-12 Thread Russell King - ARM Linux 
On Fri, Jul 12, 2013 at 05:48:13PM -0400, Santosh Shilimkar wrote:
 Most of the kernel code assumes that max*pfn is maximum pfns because
 the physical start of memory is expected to be PFN0. Since this
 assumption is not true on ARM architectures, the meaning of max*pfn
 is number of memory pages. This is done to keep drivers happy which
 are making use of of these variable to calculate the dma bounce limit
 using dma_mask.
 
 Now since we have a architecture override possibility for DMAable
 maximum pfns, lets make meaning of max*pfns as maximum pnfs on ARM
 as well.
 
 In the patch, the dma_to_pfn/pfn_to_dma() pair is hacked to take care of
 the physical memory offset. It is done this way just to enable testing
 since its understood that it can come in way of single zImage.

As Santosh says, this is a hack - but we need to have a discussion about
how to handle translations from PFN to bus addresses.

Currently, the way we do that on ARM is mostly assume that physical
addresses are the same as bus addresses, but that's not true everywhere,
and certainly isn't true when you have a 32-bit DMA controller which
can access physical memory, where the physical memory is above 4GB
in physical space.

We have certain platforms where the DMA address is already being
programmed into a controller with less than 32-bits in its address
register, and with a physical memory offset - and of course this
case just works out of the box because the high bits are ignored
by the device.

What I'm basically saying is we've had this problem for a while, and
we've lived with it by hoping and hacking, and adjusting max*pfn, but
this is not long-term sustainable.  We *need* to get away from the
idea that DMA addresses are physical addresses and device DMA masks
have some relationship to physical addresses.

Consider for a moment:

PCI address 0x --- physical address 0xc000.

You plug a card in which can't do 32-bit addressing (remember, there
are such PCI cards in the past...).  The driver sets the DMA mask to
0x0fff (or whatever).  How does that relate to the PCI bus address?
It's 0x to 0x0fff.  How does that relate to the physical
address space?  0xc000 to 0xcfff.

This is why DMA masks can't be treated as some notional address limit.
It just doesn't work when you have bus offsets.

And the extreme case of that is LPAE with all system memory above the
4GB physical mark, with 32-bit DMA capable peripherals - which we're
starting to see now.

Ideally, I think we need some kind of per-bus DT property to describe
the memory which can be accessed from the bus - to do it properly to
cover the cases we've already seen, that would be an offset and a size.

We then need some way for dma_to_pfn() and pfn_to_dma() to efficiently
get at that information - bear in mind that they're hot paths when doing
DMA mappings and the like.  I doubt we want to be looking up the same
property time and time again inside them.
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