Re: [PATCH v2 07/11] iommu: Let 32-bit ARM drivers opt into a DMA API default domain
From: Robin Murphy
Date: Tue Oct 20 2026 - 07:32:38 EST
On 16/10/2026 7:46 am, Mikko Perttunen wrote:
iommu_get_default_domain_type() unconditionally forces IDENTITY on
32-bit ARM as ARM_DMA_USE_IOMMU installs its own UNMANAGED domain on top
of whatever the group already had. Now that the ARM DMA API state is an
ordinary domain cookie, IOMMU drivers can ask for a real
IOMMU_DOMAIN_DMA default domain instead. Drop the override so that
->def_domain_type is honoured, and let a driver-requested DMA domain
through the !CONFIG_IOMMU_DMA guard when ARM_DMA_USE_IOMMU can back it.
IOMMU_DOMAIN_DMA stays strictly opt-in on 32-bit ARM and is never the
fallback to avoid unexpected changes, as drivers today return 0 from
->def_domain_type while expecting the legacy behaviour.
Behaviour is therefore unchanged for every existing 32-bit ARM IOMMU
driver:
- exynos-iommu, omap-iommu, rockchip-iommu, msm_iommu, ipmmu-vmsa,
mtk_iommu*, sun50i-iommu and qcom_iommu have no ->def_domain_type,
so iommu_get_def_domain_type() returns the unchanged cur_type of 0,
which is forced to IDENTITY as before.
- arm-smmu returns IDENTITY for legacy bindings and 0 otherwise, which
ends up in the same place.
- tegra-smmu returns IDENTITY explicitly, and keeps doing so until a
later patch.
Also move the static_assert on the two DMA API implementations being
mutually exclusive to drivers/iommu/dma-iommu.h, next to the
declarations that depend on it.
Signed-off-by: Mikko Perttunen <mperttunen@xxxxxxxxxx>
---
drivers/iommu/dma-iommu.h | 3 +++
drivers/iommu/iommu.c | 20 +++++---------------
2 files changed, 8 insertions(+), 15 deletions(-)
diff --git a/drivers/iommu/dma-iommu.h b/drivers/iommu/dma-iommu.h
index 71b23cb698c0..24fa4165450f 100644
--- a/drivers/iommu/dma-iommu.h
+++ b/drivers/iommu/dma-iommu.h
@@ -12,6 +12,9 @@
* IOMMU_DOMAIN_DMA default domains. Only one provider can be compiled in at a
* time.
*/
+static_assert(!(IS_ENABLED(CONFIG_IOMMU_DMA) &&
+ IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU)));
+
Don't the first couple of patches already make this redundant, since such a build would now fail anyway due to multiple definitions of iommu_setup_dma_ops() etc.?
#if defined(CONFIG_ARM_DMA_USE_IOMMU)
#include <asm/dma-iommu.h>
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 5fbb76d44a1e..0d812c4b9f08 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -1884,18 +1884,6 @@ static int iommu_get_default_domain_type(struct iommu_group *group,
lockdep_assert_held(&group->mutex);
- /*
- * ARM32 drivers supporting CONFIG_ARM_DMA_USE_IOMMU can declare an
- * identity_domain and it will automatically become their default
- * domain. Later on ARM_DMA_USE_IOMMU will install its UNMANAGED domain.
- * Override the selection to IDENTITY.
- */
- if (IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU)) {
- static_assert(!(IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU) &&
- IS_ENABLED(CONFIG_IOMMU_DMA)));
- driver_type = IOMMU_DOMAIN_IDENTITY;
- }
-
for_each_group_device(group, gdev) {
driver_type = iommu_get_def_domain_type(group, gdev->dev,
driver_type);
@@ -1913,11 +1901,13 @@ static int iommu_get_default_domain_type(struct iommu_group *group,
/*
* If the common dma ops are not selected in kconfig then we cannot use
- * IOMMU_DOMAIN_DMA at all. Force IDENTITY if nothing else has been
- * selected.
+ * IOMMU_DOMAIN_DMA at all, unless this is ARM32 and the driver asked
+ * for it explicitly, where CONFIG_ARM_DMA_USE_IOMMU provides the
+ * implementation. Force IDENTITY if nothing else has been selected.
*/
if (!IS_ENABLED(CONFIG_IOMMU_DMA)) {
- if (WARN_ON(driver_type == IOMMU_DOMAIN_DMA))
+ if (WARN_ON(driver_type == IOMMU_DOMAIN_DMA &&
+ !IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU)))
This ends up looking a bit odd, and not clear that it's still more or less the same special-case workaround seemingly being removed in the hunk above.
At this point I'd rather push any remaining identity special cases into the relevant drivers, however I'm somewhat more confident that there may not actually need to be any:
- exynos-iommu, omap-iommu, rockchip-iommu, msm_iommu: AFAICS these all only ever have a single client device, so the per-group implicit domain should be equivalent to the per-device explicit one, so they should not be impacted by switching (provided the "throw away the ARM domain" idiom in some of their client drivers becomes non-destructive as mentioned)
- mtk_iommu, sun50i-iommu, qcom_iommu, arm_smmu: While Kconfig might allow building them, I'm not sure we really support the first 3 on 32-bit at all, but again since these all have no special CONFIG_ARM group handling anyway then either they must work out as above or they're already broken, and thus at most the change might un-break them.
- mtk_iommu_v1 and ipmmu-vmsa probably wouldn't care given that they end up attaching to their own shared domain anyway - they just can't wait to be able to use a proper singleton group to rip all that out :D
The main reason for forcing identity was to meet the expectations of the existing ARM DMA ops setup, but if we're now touching the ARM code to properly tie that in to the IOMMU flow, then I think we can just do the right thing.
Thanks,
Robin.
return -1;
if (!driver_type)
driver_type = IOMMU_DOMAIN_IDENTITY;