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

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

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-zLv9SwRftAIdnm+yROfE0A@public.gmane.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.

Will