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Message-ID: <cfd23f75-8952-4463-abd5-815b995031b0@arm.com>
Date: Mon, 19 Jan 2026 14:13:43 +0000
From: Robin Murphy <robin.murphy@....com>
To: Vishnu Reddy <busanna.reddy@....qualcomm.com>, joro@...tes.org,
will@...nel.org, m.szyprowski@...sung.com
Cc: iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
vikash.garodia@....qualcomm.com, dikshita.agarwal@....qualcomm.com
Subject: Re: [PATCH] dma-iommu: Introduce API to reserve IOVA regions for
dynamically created devices
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@....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)
> {
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