On Wed, Nov 13, 2019 at 05:13:39PM +0100, 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. This looks sensible modulo the minor comments in this thread. > > Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx> > > --- > > Note this is rebased on top of Christoph's latest DMA series: > https://www.spinics.net/lists/arm-kernel/msg768600.html FYI, I'll plan to merge those tonight unless anyone screams.
