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

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