[PATCH] dma-mapping: treat dev->bus_dma_mask as a DMA limit

[Robin Murphy]

On 2019-11-13 8:41 pm, Florian Fainelli wrote:
> On 11/13/19 12:34 PM, Robin Murphy wrote:
>> On 13/11/2019 4:13 pm, Nicolas Saenz Julienne wrote:
>>> Using a mask to represent bus DMA constraints has a set of limitations.
>>> The biggest one being it can only hold a power of two (minus one). The
>>> DMA mapping code is already aware of this and treats dev->bus_dma_mask
>>> as a limit. This quirk is already used by some architectures although
>>> still rare.
>>>
>>> With the introduction of the Raspberry Pi 4 we've found a new contender
>>> for the use of bus DMA limits, as its PCIe bus can only address the
>>> lower 3GB of memory (of a total of 4GB). This is impossible to represent
>>> with a mask. To make things worse the device-tree code rounds non power
>>> of two bus DMA limits to the next power of two, which is unacceptable in
>>> this case.
>>>
>>> In the light of this, rename dev->bus_dma_mask to dev->bus_dma_limit all
>>> over the tree and treat it as such. Note that dev->bus_dma_limit is
>>> meant to contain the higher accesible DMA address.
>>
>> Neat, you win a "why didn't I do it that way in the first place?" :)
>>
>> Looking at it without all the history of previous attempts, this looks
>> entirely reasonable, and definitely a step in the right direction.
> 
> And while you are changing those, would it make sense to not only rename
> the structure member but introduce a getter and setter in order to ease
> future work where this would no longer be a scalar?

I doubt it - once we get as a far as supporting multiple DMA ranges, 
there will be a whole load of infrastructure churn anyway if only to 
replace dma_pfn_offset, and I'm not sure a simple get/set paradigm would 
even be viable, so it's probably better to save that until clearly 
necessary.

Robin.