[PATCH kernel] powerpc/dma: Fix dma_map_ops::get_required_mask

[Alexey Kardashevskiy]

On 08/09/2020 15:44, Christoph Hellwig wrote:
> On Tue, Sep 08, 2020 at 11:51:06AM +1000, Alexey Kardashevskiy wrote:
>> What is dma_get_required_mask() for anyway? What "requires" what here?
> 
> Yes, it is a really odd API.  It comes from classic old PCI where
> 64-bit addressing required an additional bus cycle, and various devices
> had different addressing schemes, with the smaller addresses beeing
> more efficient.  So this allows the driver to request the "required"
> addressing mode to address all memory.  "preferred" might be a better
> name as we'll bounce buffer if it isn't met.  I also don't really see
> why a driver would ever want to use it for a modern PCIe device.


a-ha, this makes more sense, thanks. Then I guess we need to revert that 
one bit from yours f1565c24b596, do not we?


-- 
Alexey