# Re: \[PATCH 2/3\] ARM/keystone: move the DMA offset handling under ifdef CONFIG\_ARM\_LPAE

- From: Russell King - ARM Linux admin \<linux@armlinux.org.uk\>
- Date: 2020-09-11 11:16:19 
- List: linux-arm-kernel
- Page: https://ratatoskr.run/linux-arm-kernel/2020/09/6101869
- Thread: https://ratatoskr.run/linux-arm-kernel/2020/09/6100011/t.md

```text
On Thu, Sep 10, 2020 at 07:40:37AM +0200, Christoph Hellwig wrote:
> The DMA offset notifier can only be used if PHYS_OFFSET is at least
> KEYSTONE_HIGH_PHYS_START, which can't be represented by a 32-bit
> phys_addr_t.  Currently the code compiles fine despite that, a pending
> change to the DMA offset handling would create a compiler warning for
> this case.  Add an ifdef to not compile the code except for LPAE
> configs.

However, to have use of the high physical offset, LPAE needs to be
enabled, which ensures that phys_addr_t is 64-bit.

I believe that DMA is non-coherent on this platform unless the high
physical address is used. Or something like that.

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