Re: [PATCH 2/3] ARM/keystone: move the DMA offset handling under ifdef CONFIG_ARM_LPAE

From: Robin Murphy
Date: Fri Sep 11 2020 - 07:32:41 EST

On 2020-09-11 12:15, Russell King - ARM Linux admin wrote:
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

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.

Yeah, it's probably not a configuration that anyone would actually want to use in anger on Keystone itself, but as long as folks might have ARCH_KEYSTONE selected in their non-LPAE multiplatform config we should avoid build regressions. I did wonder if Keystone should simply have a hard dependency on LPAE, but there does appear to be some explicit support for running directly from the non-coherent low 2G alias :/