Re: [PATCH v3 00/10] Make the 32-bit ARM DMA API work with normal IOMMU DMA default domains (+ Tegra PoC)

[Marek Szyprowski]

On 26.10.2026 10:19, Mikko Perttunen wrote:
> Depending on the probe ordering, TegraDRM does not currently probe on
> kernels with CONFIG_ARM_DMA_USE_IOMMU enabled, such as with
> multi_v7_defconfig. The Tegra SMMU puts several display and 2D/3D
> clients into one IOMMU group, and the legacy 32-bit ARM DMA API path
> creates one UNMANAGED domain per device and attaches it with
> iommu_attach_device(). That only works for one device per group, so
> every client after the first fails to attach,
> iommu_device_use_default_domain() rejects the group with -EBUSY, and the
> driver goes down with it. tegra_defconfig is currently working by virtue
> of not enabling CONFIG_ARM_DMA_USE_IOMMU.
>
> Robin suggested [1] that instead of teaching the legacy path about
> groups, we should explore modernizing the 32-bit ARM DMA code a bit by
> making it use domain cookies and have the core code own the domains, as
> is the case on other architectures. Still, ARM's own DMA code is kept to
> support existing drivers and APIs that expect it, and because dma-iommu
> is considered too big and unnecessary for 32-bit ARM.
>
> The series is in four parts:
>
> - Patches 1-2 are preparation in the IOMMU core:
>   1. Add iommu_teardown_dma_ops, counterpart to iommu_setup_dma_ops.
>   2. Add cookie type for ARM's DMA domains.
> - Patches 3-6 implement the DMA API side on 32-bit ARM:
>   3. Split up mapping alloc and init (bitmap alloc) code to separate
>      functions.
>   4. Implement parallel dma_ops / cookie APIs on ARM side.
>   5. Avoid domain allocation when IOMMU core does it.
>   6. Check upper DMA range limit during IOVA allocation.
> - Patch 7 removes the forced IDENTITY default domain in core IOMMU
>   code and adds explicit IDENTITY defaults to a few individual IOMMU
>   drivers.
> - Patches 8-10 convert Tegra:
>   8. Fixes device parameter in calls to dma_get_sgtable.
>   9. Allows deferring translation enable until driver handshake for
>      selected devices.
>   10. Switches to DMA default domain for selected Tegra devices.
>
> This moves 32-bit ARM devices from the unmanaged IOMMU DMA domain to the
> core-owned default DMA domain except for mtk_iommu_v1, ipmmu-vmsa,
> omap-iommu, and msm_iommu. (tegra-smmu also stays on the legacy path
> until the last patch in the series.) See commit 7 for further
> information. Existing drivers are intended to continue working without
> changes.
>
> Patches 8-10 are provided for demonstration; we can deal with them 
> separately if the rest of the series is accepted.
>
> Patches 1,2,3,6 are applicable independently.
> Patches 4,5 depend on the previous patches.
> Patch 7 requires the prior patches to be in place.
>
> Patches 1-7 only touch drivers/iommu/ and arch/arm/mm/ and associated 
> headers.


I've tested this patchset on ARM 32bit based Exynos boards and it works
fine, although I get the following warnings during boot:

platform 13400000.codec: Adding to iommu group 0
iommu: Failed to allocate default IOMMU domain of type 0 for group (null) - Falling back to IOMMU_DOMAIN_DMA
platform 11840000.jpeg-codec: Adding to iommu group 1
iommu: Failed to allocate default IOMMU domain of type 0 for group (null) - Falling back to IOMMU_DOMAIN_DMA

This looks like an issue in iommu core, which doesn't expect devices with
iommu to come up so early.


LLM proposes the following fix for it:

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 197a8f70c6de..612933306684 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -225,10 +225,8 @@ static const char *iommu_domain_type_str(unsigned int t)
        }
 }

-static int __init iommu_subsys_init(void)
+static void __init iommu_init_def_domain_type(void)
 {
-       struct notifier_block *nb;
-
        if (!(iommu_cmd_line & IOMMU_CMD_LINE_DMA_API)) {
                if (IS_ENABLED(CONFIG_IOMMU_DEFAULT_PASSTHROUGH))
                        iommu_set_default_passthrough(false);
@@ -254,6 +252,11 @@ static int __init iommu_subsys_init(void)
                        iommu_dma_strict ? "strict" : "lazy",
                        (iommu_cmd_line & IOMMU_CMD_LINE_STRICT) ?
                                " (set via kernel command line)" : "");
+}
+
+static int __init iommu_subsys_init(void)
+{
+       struct notifier_block *nb;

        nb = kzalloc_objs(*nb, ARRAY_SIZE(iommu_buses));
        if (!nb)
@@ -2939,6 +2942,12 @@ EXPORT_SYMBOL_GPL(report_iommu_fault);

 static int __init iommu_init(void)
 {
+       /*
+        * Select the default domain type before any IOMMU driver can register
+        * and create groups, which may happen before subsys_initcall.
+        */
+       iommu_init_def_domain_type();
+
        iommu_group_kset = kset_create_and_add("iommu_groups",
                                               NULL, kernel_kobj);
        BUG_ON(!iommu_group_kset);

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland