Re: [PATCH] ata: ahci_platform: add 32-bit quirk for dwc-ahci

[Robin Murphy <robin.murphy@arm.com>]

On 2020-02-12 11:56 am, David Laight wrote:
> From: Robin Murphy
>> Sent: 12 February 2020 11:43
> ...
>> If the device *is* inherently 64-bit capable, then setting 64-bit masks
>> in the driver is correct - if a 64-bit IP block happens to have been
>> integrated with only 32 address bits wired up, but the system has memory
>> above the 32-bit boundary, then that should be described via
>> "dma-ranges", which should then end up being used to further constrain
>> the device masks internally to the DMA API.
> 
> Given how often this happens (please can I shoot some more
> hardware engineers - he says while compiling some VHDL)
> is it possible to allocate some memory pages that are
> aliases if the address bits over 31 are ignored?
> 
> Then (at least some) drivers could to a run-time probe
> reading to the high address and checking the data didn't
> appear in the low address.
> 
> Only one such set of pages is needed - access can be locked.
> But they'd need to be reserved early on.

(Oh, for that much control over page allocation!)

It's a fun idea, but there's no guarantee that the platform memory map 
is actually suitable (AMD Seattle is the go-to counterexample), and 
having an opt-in test that every driver has to implement individually 
sounds like a maintenance nightmare. I think it's far better all round 
to just expect the firmware-provided machine description - be it 
devicetree, ACPI, or whatever - to correctly describe the integration.

Robin.