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

Nicolas Saenz Julienne
2019-11-21T15:26:09Z

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On Thu, 2019-11-21 at 16:24 +0100, Christoph Hellwig wrote:
> 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.

Thanks!

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