Re: [PATCH v2 07/11] iommu: Let 32-bit ARM drivers opt into a DMA API default domain

[Mikko Perttunen <mperttunen@nvidia.com>]

On Tuesday, October 20, 2026 8:28 PM Robin Murphy wrote:
> 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@nvidia.com>
> > ---
> >   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.?

That is true. I'll get rid of this.

> 
> >   #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.

Very well, I'll give it a shot! You're right that it /should/ work :)

Thanks
Mikko

> 
> >   			return -1;
> >   		if (!driver_type)
> >   			driver_type = IOMMU_DOMAIN_IDENTITY;
> > 
> 
>