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

Robin Murphy
2019-11-21T15:55:50Z

On 21/11/2019 3:26 pm, Christoph Hellwig wrote:
> On Thu, Nov 21, 2019 at 04:24:57PM +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.
>
> Actually. This doesn't apply because the dma-mapping tree doesn't
> have you zone_dma_bits change. I guess we'll need to wait for the
> next merge window, or maybe post rc1 if this happens to fix the
> powerpc problem that Christian reported.

Hmm, there's no functional dependency though, is there? AFAICS it's
essentially just a context conflict. Is it worth simply dropping (or
postponing) the local renaming in __dma_direct_optimal_gfp_mask(), or
perhaps even cross-merging arm64/for-next/zone-dma into dma/for-next?

Robin.