[PATCH v3 2/2] drm/nouveau: tegra: Detach from ARM DMA/IOMMU mapping

[Thierry Reding]

On Wed, May 30, 2018 at 02:30:51PM +0100, Robin Murphy wrote:
> On 30/05/18 14:00, Thierry Reding wrote:
> > On Wed, May 30, 2018 at 11:30:25AM +0100, Robin Murphy wrote:
> > > On 30/05/18 09:03, Thierry Reding wrote:
> > > > From: Thierry Reding <treding at nvidia.com>
> > > > 
> > > > Depending on the kernel configuration, early ARM architecture setup code
> > > > may have attached the GPU to a DMA/IOMMU mapping that transparently uses
> > > > the IOMMU to back the DMA API. Tegra requires special handling for IOMMU
> > > > backed buffers (a special bit in the GPU's MMU page tables indicates the
> > > > memory path to take: via the SMMU or directly to the memory controller).
> > > > Transparently backing DMA memory with an IOMMU prevents Nouveau from
> > > > properly handling such memory accesses and causes memory access faults.
> > > > 
> > > > As a side-note: buffers other than those allocated in instance memory
> > > > don't need to be physically contiguous from the GPU's perspective since
> > > > the GPU can map them into contiguous buffers using its own MMU. Mapping
> > > > these buffers through the IOMMU is unnecessary and will even lead to
> > > > performance degradation because of the additional translation. One
> > > > exception to this are compressible buffers which need large pages. In
> > > > order to enable these large pages, multiple small pages will have to be
> > > > combined into one large (I/O virtually contiguous) mapping via the
> > > > IOMMU. However, that is a topic outside the scope of this fix and isn't
> > > > currently supported. An implementation will want to explicitly create
> > > > these large pages in the Nouveau driver, so detaching from a DMA/IOMMU
> > > > mapping would still be required.
> > > 
> > > I wonder if it might make sense to have a hook in iommu_attach_device() to
> > > notify the arch DMA API code when moving devices between unmanaged and DMA
> > > ops domains? That seems like it might be the most low-impact way to address
> > > the overall problem long-term.
> > > 
> > > > Signed-off-by: Thierry Reding <treding at nvidia.com>
> > > > ---
> > > > Changes in v3:
> > > > - clarify the use of IOMMU mapping for compressible buffers
> > > > - squash multiple patches into this
> > > > 
> > > >    drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c | 5 +++++
> > > >    1 file changed, 5 insertions(+)
> > > > 
> > > > diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c b/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c
> > > > index 78597da6313a..d0538af1b967 100644
> > > > --- a/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c
> > > > +++ b/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c
> > > > @@ -105,6 +105,11 @@ nvkm_device_tegra_probe_iommu(struct nvkm_device_tegra *tdev)
> > > >    	unsigned long pgsize_bitmap;
> > > >    	int ret;
> > > > +#if IS_ENABLED(CONFIG_ARM)
> > > 
> > > Wouldn't CONFIG_ARM_DMA_USE_IOMMU be even more appropriate?
> > 
> > Not necessarily. arm_dma_iommu_detach_device() is always defined on ARM,
> > only with CONFIG_ARM_DMA_USE_IOMMU=n it will be empty. So this check is
> > a guard to make sure we don't call the function when it isn't available,
> > but it may still not do anything.
> 
> Calling a function under condition A, which only does anything under
> condition B, when B depends on A, is identical in behaviour to only calling
> the function under condition B, except needlessly harder to follow.
> 
> > > > +	/* make sure we can use the IOMMU exclusively */
> > > > +	arm_dma_iommu_detach_device(dev);
> > > 
> > > As before, I would just use the existing infrastructure the same way the
> > > Exynos DRM driver currently does in __exynos_iommu_attach() (albeit without
> > > then reattaching to another DMA ops mapping).
> > 
> > That's pretty much what I initially did and which was shot down by
> > Christoph. As I said earlier, at this point I don't really care what
> > color the shed will be. Can you and Christoph come to an agreement
> > on what it should be?
> 
> What I was getting at is that arm_iommu_detach_device() already *is* the
> exact function Christoph was asking for, it just needs a minor fix instead
> of adding explicit set_dma_ops() fiddling at its callsites which only
> obfuscates the fact that it's supposed to be responsible for resetting the
> device's DMA ops already.

It still has the downside of callers having to explicitly check for the
existence of a mapping, otherwise they'll cause a warning to be printed
to the kernel log.

That's not all that bad, though. I'll prepare version 4 with those
changes.

Thierry
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