Re: [PATCH 16/18] staging/media/tegra-vde: Clean up IOMMU workaround

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On 2020-08-20 20:51, Dmitry Osipenko wrote:
20.08.2020 18:08, Robin Murphy пишет:
Now that arch/arm is wired up for default domains and iommu-dma, we no
longer need to work around the arch-private mapping.

Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---
  drivers/staging/media/tegra-vde/iommu.c | 12 ------------
  1 file changed, 12 deletions(-)

diff --git a/drivers/staging/media/tegra-vde/iommu.c b/drivers/staging/media/tegra-vde/iommu.c
index 6af863d92123..4f770189ed34 100644
--- a/drivers/staging/media/tegra-vde/iommu.c
+++ b/drivers/staging/media/tegra-vde/iommu.c
@@ -10,10 +10,6 @@
  #include <linux/kernel.h>
  #include <linux/platform_device.h>
-#if IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU)
-#include <asm/dma-iommu.h>
-#endif
-
  #include "vde.h"
int tegra_vde_iommu_map(struct tegra_vde *vde,
@@ -70,14 +66,6 @@ int tegra_vde_iommu_init(struct tegra_vde *vde)
  	if (!vde->group)
  		return 0;
-#if IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU)
-	if (dev->archdata.mapping) {
-		struct dma_iommu_mapping *mapping = to_dma_iommu_mapping(dev);
-
-		arm_iommu_detach_device(dev);
-		arm_iommu_release_mapping(mapping);
-	}
-#endif
  	vde->domain = iommu_domain_alloc(&platform_bus_type);
  	if (!vde->domain) {
  		err = -ENOMEM;


Hello, Robin! Thank you for yours work!

Some drivers, like this Tegra VDE (Video Decoder Engine) driver for
example, do not want to use implicit IOMMU domain.

That isn't (intentionally) changing here - the only difference should be that instead of having the ARM-special implicit domain, which you have to kick out of the way with the ARM-specific API before you're able to attach your own domain, the implicit domain is now a proper IOMMU API default domain, which automatically gets bumped by your attach. The default domains should still only be created in the same cases that the ARM dma_iommu_mappings were.

Tegra VDE driver
relies on explicit IOMMU domain in a case of Tegra SMMU because VDE
hardware can't access last page of the AS and because driver wants to
reserve some fixed addresses [1].

[1]
https://elixir.bootlin.com/linux/v5.9-rc1/source/drivers/staging/media/tegra-vde/iommu.c#L100

Tegra30 SoC supports up to 4 domains, hence it's not possible to afford
wasting unused implicit domains. I think this needs to be addressed
before this patch could be applied.

Yeah, there is one subtle change in behaviour from removing the ARM layer on top of the core API, in that the IOMMU driver will no longer see an explicit detach call. Thus it does stand to benefit from being a bit cleverer about noticing devices being moved from one domain to another by an attach call, either by releasing the hardware context for the inactive domain once the device(s) are moved across to the new one, or by simply reprogramming the hardware context in-place for the new domain's address space without allocating a new one at all (most of the drivers that don't have multiple contexts already handle the latter approach quite well).

Would it be possible for IOMMU drivers to gain support for filtering out
devices in iommu_domain_alloc(dev, type)? Then perhaps Tegra SMMU driver
could simply return NULL in a case of type=IOMMU_DOMAIN_DMA and
dev=tegra-vde.

If you can implement IOMMU_DOMAIN_IDENTITY by allowing the relevant devices to bypass translation entirely without needing a hardware context (or at worst, can spare one context which all identity-mapped logical domains can share), then you could certainly do that kind of filtering with the .def_domain_type callback if you really wanted to. As above, the intent is that that shouldn't be necessary for this particular case, since only one of a group's default domain and explicitly attached domain can be live at any given time, so the driver should be able to take advantage of that.

If you simply have more active devices (groups) than available contexts then yes, you probably would want to do some filtering to decide who deserves a translation domain and who doesn't, but in that case you should already have had a long-standing problem with the ARM implicit domains.

Alternatively, the Tegra SMMU could be changed such that the devices
will be attached to a domain at the time of a first IOMMU mapping
invocation instead of attaching at the time of attach_dev() callback
invocation.

Or maybe even IOMMU core could be changed to attach devices at the time
of the first IOMMU mapping invocation? This could be a universal
solution for all drivers.

I suppose technically you could do that within an IOMMU driver already (similar to how some defer most of setup that logically belongs to ->domain_alloc until the first ->attach_dev). It's a bit grim from the caller's PoV though, in terms of the failure mode being non-obvious and having no real way to recover. Again, you'd be better off simply making decisions up-front at domain_alloc or attach time based on the domain type.

Robin.
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