Re: [PATCH v2] dma-mapping: treat dev->bus_dma_mask as a DMA limit

Christoph Hellwig
2019-11-21T15:25:07Z

On Thu, Nov 21, 2019 at 10:26:44AM +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 should
> contain the higher accesible DMA address.
>
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>

I've tentatively added this patch to the dma-mapping tree based on
Robins principal approval of the last version. That way tomorrows
linux-next run should still pick it up.