Re: [PATCH] [RFC] gpu: host1x: shut up warning about DMA API misuse

[Arnd Bergmann]

On Thu, Apr 20, 2017 at 11:44 AM, Mikko Perttunen <cyndis@xxxxxxxx> wrote:
> On 20.04.2017 11:25, Arnd Bergmann wrote:
>> On Thu, Apr 20, 2017 at 9:02 AM, Mikko Perttunen <cyndis@xxxxxxxx> wrote:
>>> On 19.04.2017 21:24, Arnd Bergmann wrote:
>>
>> I don't think this can be a per-platform policy.
>
>
> Yeah, now that we are using the ARM SMMU on T186 onwards it's more difficult
> than when we were using the Tegra SMMU, so we'll probably have to change
> that.
>
> The goal of the current code is to allocate memory from the CMA, so one
> option would be to change it to use dma_alloc_from_contiguous. That way we
> wouldn't get the double IOMMU mapping, which would be nice.

Right, also we shouldn't define what a particular API does based on
which platform you run on.

> The goal of the current code is to allocate memory from the CMA, so one
> option would be to change it to use dma_alloc_from_contiguous. That way
> we wouldn't get the double IOMMU mapping, which would be nice.

dma_alloc_from_contiguous() is intentionally not exported to drivers,
and it would result in a page that is not mapped into your kernel
address space.

This is probably not the only driver that has this issue, so maybe a
better approach would be to fill the API gap and introduce an IOMMU
API function that takes a domain/iova/length tuple as its argument
and allocates coherent or WC memory that it maps into that address?

      Arnd