On Wed, 2019-11-13 at 17:13 +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.

Signed-off-by: Nicolas Saenz Julienne <redacted>

Hi Rob & Christoph,
do you mind if I append v2 of this into my upcoming v3 RPi4 PCIe support
series, I didn't do it initially as I thought this was going to be a
contentious patch.  But as it turned out better than expected, I think it
should go into the PCIe series. In the end it's the first explicit user of the
bus DMA limit.

Here's v2 in case you don't know what I'm talking about:
https://www.spinics.net/lists/arm-kernel/msg768459.html

Regards,
Nicolas