[PATCH v1] iommu/riscv: Support 32-bit register accesses

* [PATCH v1] iommu/riscv: Support 32-bit register accesses
@ 2026-06-15  6:48 ` Zhanpeng Zhang
  0 siblings, 0 replies; 70+ messages in thread
From: Zhanpeng Zhang @ 2026-06-15  6:48 UTC (permalink / raw)
  To: Tomasz Jeznach, Joerg Roedel, Will Deacon
  Cc: Robin Murphy, Paul Walmsley, Palmer Dabbelt, Albert Ou,
	Alexandre Ghiti, iommu, linux-riscv, linux-kernel, Xu Lu,
	cuiyunhui, yuanzhu, Zhanpeng Zhang

Some RISC-V IOMMU implementations cannot perform 64-bit MMIO accesses
to the IOMMU register file. The RISC-V IOMMU architecture allows 64-bit
registers to be accessed using 32-bit accesses, provided the accesses are
properly aligned and do not span multiple registers.

Add a config option for such implementations and access 64-bit IOMMU
registers as paired 32-bit MMIO operations when it is enabled. Serialize
the paired accesses so the high and low halves cannot interleave with
another CPU. Full 64-bit register programming writes the high half before
the low half.

This option describes the register access width. It is not an RV32 kernel
mode and does not describe a 32-bit IOMMU architecture.

Co-developed-by: Xu Lu <luxu.kernel@bytedance.com>
Signed-off-by: Xu Lu <luxu.kernel@bytedance.com>
Signed-off-by: Zhanpeng Zhang <zhangzhanpeng.jasper@bytedance.com>
---
This is needed for platforms whose RISC-V IOMMU register window does not
support naturally aligned 64-bit MMIO accesses.

 drivers/iommu/riscv/Kconfig | 11 ++++++++
 drivers/iommu/riscv/iommu.c |  4 +++
 drivers/iommu/riscv/iommu.h | 55 +++++++++++++++++++++++++++++++++----
 3 files changed, 64 insertions(+), 6 deletions(-)

diff --git a/drivers/iommu/riscv/Kconfig b/drivers/iommu/riscv/Kconfig
index b86e5ab94183..54d624b9b2ef 100644
--- a/drivers/iommu/riscv/Kconfig
+++ b/drivers/iommu/riscv/Kconfig
@@ -22,3 +22,14 @@ config RISCV_IOMMU_PCI
 	def_bool y if RISCV_IOMMU && PCI_MSI
 	help
 	  Support for the PCIe implementation of RISC-V IOMMU architecture.
+
+config RISCV_IOMMU_32BIT_ACCESS
+	bool "Use 32-bit accesses for RISC-V IOMMU registers"
+	depends on RISCV_IOMMU
+	help
+	  Say Y when the RISC-V IOMMU MMIO window cannot be accessed
+	  using naturally aligned 64-bit loads and stores.
+
+	  When enabled, 64-bit IOMMU registers are accessed as paired
+	  32-bit MMIO operations. This option does not describe an RV32
+	  kernel or a 32-bit IOMMU architecture.
diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
index a31f50bbad35..7fa1721b5728 100644
--- a/drivers/iommu/riscv/iommu.c
+++ b/drivers/iommu/riscv/iommu.c
@@ -53,6 +53,10 @@ struct riscv_iommu_devres {
 	void *addr;
 };
 
+#ifdef CONFIG_RISCV_IOMMU_32BIT_ACCESS
+DEFINE_RAW_SPINLOCK(riscv_iommu_32bit_access_lock);
+#endif
+
 static void riscv_iommu_devres_pages_release(struct device *dev, void *res)
 {
 	struct riscv_iommu_devres *devres = res;
diff --git a/drivers/iommu/riscv/iommu.h b/drivers/iommu/riscv/iommu.h
index 46df79dd5495..ba78ef1858c5 100644
--- a/drivers/iommu/riscv/iommu.h
+++ b/drivers/iommu/riscv/iommu.h
@@ -14,6 +14,9 @@
 #include <linux/iommu.h>
 #include <linux/types.h>
 #include <linux/iopoll.h>
+#ifdef CONFIG_RISCV_IOMMU_32BIT_ACCESS
+#include <linux/spinlock.h>
+#endif
 
 #include "iommu-bits.h"
 
@@ -69,21 +72,61 @@ void riscv_iommu_disable(struct riscv_iommu_device *iommu);
 #define riscv_iommu_readl(iommu, addr) \
 	readl_relaxed((iommu)->reg + (addr))
 
-#define riscv_iommu_readq(iommu, addr) \
-	readq_relaxed((iommu)->reg + (addr))
-
 #define riscv_iommu_writel(iommu, addr, val) \
 	writel_relaxed((val), (iommu)->reg + (addr))
 
+#define riscv_iommu_readl_timeout(iommu, addr, val, cond, delay_us, timeout_us) \
+	readx_poll_timeout(readl_relaxed, (iommu)->reg + (addr), val, cond, \
+			   delay_us, timeout_us)
+
+#ifndef CONFIG_RISCV_IOMMU_32BIT_ACCESS
+#define riscv_iommu_readq(iommu, addr) \
+	readq_relaxed((iommu)->reg + (addr))
+
 #define riscv_iommu_writeq(iommu, addr, val) \
 	writeq_relaxed((val), (iommu)->reg + (addr))
 
 #define riscv_iommu_readq_timeout(iommu, addr, val, cond, delay_us, timeout_us) \
 	readx_poll_timeout(readq_relaxed, (iommu)->reg + (addr), val, cond, \
 			   delay_us, timeout_us)
+#else /* CONFIG_RISCV_IOMMU_32BIT_ACCESS */
 
-#define riscv_iommu_readl_timeout(iommu, addr, val, cond, delay_us, timeout_us) \
-	readx_poll_timeout(readl_relaxed, (iommu)->reg + (addr), val, cond, \
-			   delay_us, timeout_us)
+extern raw_spinlock_t riscv_iommu_32bit_access_lock;
+
+static inline u64 __riscv_iommu_readq_relaxed(void __iomem *addr)
+{
+	u32 lo, hi;
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&riscv_iommu_32bit_access_lock, flags);
+	do {
+		hi = readl_relaxed(addr + sizeof(u32));
+		lo = readl_relaxed(addr);
+	} while (hi != readl_relaxed(addr + sizeof(u32)));
+	raw_spin_unlock_irqrestore(&riscv_iommu_32bit_access_lock, flags);
+
+	return ((u64)hi << 32) | (u64)lo;
+}
+
+static inline void __riscv_iommu_writeq_relaxed(u64 value, void __iomem *addr)
+{
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&riscv_iommu_32bit_access_lock, flags);
+	writel_relaxed((u32)(value >> 32), addr + sizeof(u32));
+	writel_relaxed((u32)value, addr);
+	raw_spin_unlock_irqrestore(&riscv_iommu_32bit_access_lock, flags);
+}
+
+#define riscv_iommu_readq(iommu, addr) \
+	__riscv_iommu_readq_relaxed((iommu)->reg + (addr))
+
+#define riscv_iommu_writeq(iommu, addr, val) \
+	__riscv_iommu_writeq_relaxed((val), (iommu)->reg + (addr))
+
+#define riscv_iommu_readq_timeout(iommu, addr, val, cond, delay_us, timeout_us) \
+	readx_poll_timeout(__riscv_iommu_readq_relaxed, (iommu)->reg + (addr), \
+			   val, cond, delay_us, timeout_us)
+#endif /* CONFIG_RISCV_IOMMU_32BIT_ACCESS */
 
 #endif
-- 
2.50.1 (Apple Git-155)

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