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

[Rob Clark]

On Thu, Feb 2, 2017 at 10:12 AM, Will Deacon <will.deacon at arm.com> wrote:
> On Thu, Feb 02, 2017 at 10:02:50AM -0500, Rob Clark wrote:
>> 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..
>
> I still need to understand the unmanaged domain case, but I don't really
> see why that's related to this series to be honest.
>

Only relation is if we were trying to solve the case of a driver that
needs to manage it's own domain with this patchset..  but that seems a
bit like "when all you have is a hammer, everything looks like a
nail"..

BR,
-R