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
