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

[Robin Murphy]

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
> > 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.

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 :/

Robin.