On 30/05/18 14:41, Thierry Reding wrote:
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@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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.
Or we could look at the way it's actually used, and reconsider whether
the warning is really appropriate. That's always an option ;)
Robin.
That's not all that bad, though. I'll prepare version 4 with those
changes.
Thierry
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