[PATCH 0/5] Implement SMMU passthrough using the default domain

[Rob Clark]

On Thu, Jan 26, 2017 at 12:18 PM, Joerg Roedel <joro at 8bytes.org> wrote:
> On Tue, Jan 24, 2017 at 08:42:23PM +0530, Sricharan wrote:
>> Thanks for this series. We had a case with the GPU.
>> The GPU's iommu was setup by kernel and the GPU
>> also does dynamic updates for on-the-fly switching between
>> process pagetables.  GPU driver was not using DMA domain and
>> the GPU's firmware was always expecting to run out  of contextbank
>>  '0' (although not correct) , which was not the case after the DMA domain
>> was made default  as '0' was getting allocated for DMA domain and
>> there were concerns about reusing the DMA domain as well.
>> Now with this series, looks there is an way out of that that can be tried.
>>
>> So should the default domain not be per device specific selectable ?
>
> Note that iommu-drivers can request direct-mapping for any given device
> on its initializtion. This is used on x86 for devices that need a 1-1
> mapping for some reason.
>
> Also device drivers can use the iommu-api and assign their own domain to
> a device, which allows them to manage the dma address space on their
> own.

Part of the problem is that dev->archdata.dma_ops gets wired up to
iommu_dma_ops.  Which isn't so bad on it's own, except that cache ops
are not exposed to drivers, forcing us to use dma-mapping API
(dma_map_sg, etc) for cache operations.

Possibly we should just expose cache op's to drivers bypass this abuse
of dma-mapping.

btw, Will, we definitely want this to *not* rely on kcmdline for the
gpu with it's own private iommu case..

BR,
-R