arm64: support DMA zone above 4GB

Commit 791ab8b ("arm64: Ignore any DMA offsets in the
max_zone_phys() calculation") made arm64 DMA/DMA32 zones span the entire
RAM when RAM starts above 32-bits. This breaks hardware with DMA area
that start above 32-bits. But the commit log says that "we haven't
noticed any such hardware". It turns out that such hardware does exist.

One such platform has RAM starting at 32GB with an internal bus that has
the following DMA limits:

  #address-cells = <2>;
  #size-cells = <2>;
  dma-ranges = <0x00 0xc0000000 0x08 0x00000000 0x00 0x40000000>;

That is, devices under this bus see 1GB of DMA range between 3GB-4GB in
their address space. This range is mapped to CPU memory at 32GB-33GB.
With current code DMA allocations for devices under this bus are not
limited to DMA area, leading to run-time allocation failure.

This commit reinstates DMA zone at the bottom of RAM. The result is DMA
zone that properly reflects the hardware constraints as follows:

[    0.000000] Zone ranges:
[    0.000000]   DMA      [mem 0x0000000800000000-0x000000083fffffff]
[    0.000000]   DMA32    empty
[    0.000000]   Normal   [mem 0x0000000840000000-0x0000000bffffffff]

Signed-off-by: Catalin Marinas <[email protected]>
[baruch: split off the original patch]
Signed-off-by: Baruch Siach <[email protected]>
Reviewed-by: Petr Tesarik <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>

Author: ctmarinas

arch/arm64/mm/init.c

@@ -114,20 +114,8 @@ static void __init arch_reserve_crashkernel(void)
 				    low_size, high);
 }
 
-/*
- * Return the maximum physical address for a zone given its limit.
- * If DRAM starts above 32-bit, expand the zone to the maximum
- * available memory, otherwise cap it at 32-bit.
- */
 static phys_addr_t __init max_zone_phys(phys_addr_t zone_limit)
 {
-	phys_addr_t phys_start = memblock_start_of_DRAM();
-
-	if (phys_start > U32_MAX)
-		zone_limit = PHYS_ADDR_MAX;
-	else if (phys_start > zone_limit)
-		zone_limit = U32_MAX;
-
 	return min(zone_limit, memblock_end_of_DRAM() - 1) + 1;
 }