Re: [PATCH] dma-iommu: Introduce API to reserve IOVA regions for dynamically created devices

[Robin Murphy <robin.murphy@arm.com>]

On 19/01/2026 5:49 am, Vishnu Reddy wrote:
> Drivers can create child devices dynamically. These child devices often
> inherit parent device tree node, including its reserved iommu-address
> ranges.

Which frankly sounds entirely wrong. If there are distinct devices with 
distinct hardware properties, then they should be properly described as 
such, and handled by a proper bus driver. And if different devices have 
different DMA restrictions, that should be described by "dma-ranges", 
not by abusing reserved regions.

Thanks,
Robin.

> While device tree can declare the multiple iommu-address ranges,
> they still apply uniformly to any child inheriting the parent OF node.
> 
> This creates a limitation when child requires a different iommu-address
> range than its parent, as device tree cannot express the reservation per
> child.
> 
> Example layout and use case:
> ----------------------------
> A multimedia subsystem with multiple child devices and distinct ranges:
> 
>      +-----------------------------------------------------------+
>      | Device A reserved region (256 MB)                         |
>      | 0x00000000 - 0x10000000                                   |
>      +-----------------------------------------------------------+
>      | Device B reserved region (512 MB)                         |
>      | 0x00000000 - 0x20000000                                   |
>      +-----------------------------------------------------------+
>      | Device C reserved region (600 MB)                         |
>      | 0x00000000 - 0x25800000                                   |
>      +-----------------------------------------------------------+
>      | Other reserved regions                                    |
>      +-----------------------------------------------------------+
> 
> If we add Device C's reserved range (0x0 to 0x025800000) into iommu
> address range into device tree and assign that OF node to Device B
> and C, it wastes memory:
> - Device A wastes ~344 MB
> - Device B wastes ~88 MB
> 
> To avoid this, drivers can use the introduced API to set their device
> specific IOVA region at runtime, ensuring the DMA mapping without
> unnecessary address space reservation.
> 
> Signed-off-by: Vishnu Reddy <busanna.reddy@oss.qualcomm.com>
> Link: https://lore.kernel.org/all/20251220023555.3017456-1-busanna.reddy@oss.qualcomm.com/
> ---
>   drivers/iommu/dma-iommu.c   | 44 +++++++++++++++++++++++++++++++++++++
>   include/linux/dma-mapping.h |  7 ++++++
>   2 files changed, 51 insertions(+)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index aeaf8fad985c3..baee119e1c277 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1743,6 +1743,50 @@ size_t iommu_dma_max_mapping_size(struct device *dev)
>   	return SIZE_MAX;
>   }
>   
> +/**
> + * dma_iova_set_resv_region - Set a reserved region in the IOVA space
> + * @dev: Device to set the reserved region for
> + * @start: Start of the reserved region
> + * @length: Length of the reserved region
> + *
> + * This function enables drivers to set device specific reservations at
> + * runtime, which is particularly useful for dynamically created child
> + * devices that requires distinct IOVA ranges than parent.
> + *
> + * Returns:
> + *  0        - Success
> + *  -ENODEV  - No valid IOMMU DMA domain for the device
> + *  -EINVAL  - Invalid domain or cookie type for this operation
> + *  -ENOMEM  - Failed to reserve the requested range
> + */
> +int dma_iova_set_resv_region(struct device *dev, unsigned long start,
> +			     unsigned long length)
> +{
> +	struct iommu_dma_cookie *cookie;
> +	struct iommu_domain *domain;
> +	struct iova_domain *iovad;
> +	unsigned long lo, hi;
> +
> +	domain = iommu_get_dma_domain(dev);
> +	if (!domain || domain->type != IOMMU_DOMAIN_DMA)
> +		return -ENODEV;
> +
> +	cookie = domain->iova_cookie;
> +	if (!cookie || domain->cookie_type != IOMMU_COOKIE_DMA_IOVA)
> +		return -EINVAL;
> +
> +	iovad = &cookie->iovad;
> +
> +	lo = iova_pfn(iovad, start);
> +	hi = iova_pfn(iovad, start + length - 1);
> +
> +	if (!reserve_iova(iovad, lo, hi))
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(dma_iova_set_resv_region);
> +
>   /**
>    * dma_iova_try_alloc - Try to allocate an IOVA space
>    * @dev: Device to allocate the IOVA space for
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 29973baa05816..75b65160a37f3 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -347,6 +347,8 @@ static inline bool dma_use_iova(struct dma_iova_state *state)
>   	return state->__size != 0;
>   }
>   
> +int dma_iova_set_resv_region(struct device *dev, unsigned long start,
> +			     unsigned long length);
>   bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state,
>   		phys_addr_t phys, size_t size);
>   void dma_iova_free(struct device *dev, struct dma_iova_state *state);
> @@ -366,6 +368,11 @@ static inline bool dma_use_iova(struct dma_iova_state *state)
>   {
>   	return false;
>   }
> +static inline int dma_iova_set_resv_region(struct device *dev, unsigned long start,
> +					   unsigned long length)
> +{
> +	return -ENODEV;
> +}
>   static inline bool dma_iova_try_alloc(struct device *dev,
>   		struct dma_iova_state *state, phys_addr_t phys, size_t size)
>   {