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

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Message-Id: <20251220023555.3017456-1-busanna.reddy@oss.qualcomm.com>
Date: Sat, 20 Dec 2025 08:05:55 +0530
From: Vishnu Reddy <busanna.reddy@....qualcomm.com>
To: robin.murphy@....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,
        Vishnu Reddy <busanna.reddy@....qualcomm.com>
Subject: [RFC PATCH] dma-iommu: Introduce API to reserve IOVA regions for dynamically created devices

Drivers can create child devices dynamically. These child devices often
inherit parent device tree node, including its reserved iommu-address
ranges. 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>
---
 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 c92088855450a..02c54cddf9417 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 aa36a0d1d9df6..c89147cf92231 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -339,6 +339,8 @@ 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);
@@ -358,6 +360,11 @@ 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)
 {
-- 
2.34.1