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

[Christoph Hellwig]

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.