[PATCH v3] iommu/riscv: Use 32-bit MMIO accesses for 64-bit registers

[Zhanpeng Zhang]

The RISC-V IOMMU specification [1] permits 64-bit registers to be accessed
using two 32-bit transactions, high half first, and leaves the single-copy
atomicity of 8-byte IOMMU register accesses unspecified.

Use the generic hi_lo_readq_relaxed() and hi_lo_writeq_relaxed() helpers
for ordinary 64-bit IOMMU registers. For DDTP, poll BUSY in the low half,
then read the high half and compose the register value from the polled low
half. HPM counter reads require a rollover-aware sequence and remain
outside these accessors.

This follows the 32-bit access direction proposed by Guo Ren [2] and uses
the generic non-atomic MMIO helpers suggested by David Laight.

[1] https://docs.riscv.org/reference/iommu/
[2] https://lore.kernel.org/r/20250903144217.837448-1-guoren@kernel.org

Suggested-by: Guo Ren <guoren at kernel.org>
Suggested-by: David Laight <david.laight.linux at gmail.com>
Signed-off-by: Zhanpeng Zhang <zhangzhanpeng.jasper at bytedance.com>
---
Changes in v3:
- Use the DDTP access sequence from [1]: retain the low half returned by
  BUSY polling, read only the high half, and compose the DDTP value from
  those two 32-bit reads.

Changes in v2:
- Rework the patch based on Guo Ren's earlier proposal [1].
- Drop the build-time option and use 32-bit accesses unconditionally.
- Drop the global lock and use the generic high-low MMIO helpers, as
  suggested by David Laight.
- Poll DDTP.BUSY through its low half.

Link to v1: [2]
Specification discussion: [3]

[1]: https://lore.kernel.org/r/20250903144217.837448-1-guoren@kernel.org
[2]: https://lore.kernel.org/r/20260615064855.90316-1-zhangzhanpeng.jasper@bytedance.com
[3]: https://github.com/riscv-non-isa/riscv-iommu/issues/765

 drivers/iommu/riscv/iommu.c | 9 ++++++---
 drivers/iommu/riscv/iommu.h | 9 +++------
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
index cec3ddd7ab1..d647b71ebec 100644
--- a/drivers/iommu/riscv/iommu.c
+++ b/drivers/iommu/riscv/iommu.c
@@ -670,9 +670,12 @@ void riscv_iommu_disable(struct riscv_iommu_device *iommu)
 
 #define riscv_iommu_read_ddtp(iommu) ({ \
 	u64 ddtp; \
-	riscv_iommu_readq_timeout((iommu), RISCV_IOMMU_REG_DDTP, ddtp, \
-				  !(ddtp & RISCV_IOMMU_DDTP_BUSY), 10, \
+	u32 ddtp_lo, ddtp_hi; \
+	riscv_iommu_readl_timeout((iommu), RISCV_IOMMU_REG_DDTP, ddtp_lo, \
+				  !(ddtp_lo & RISCV_IOMMU_DDTP_BUSY), 10, \
 				  RISCV_IOMMU_DDTP_TIMEOUT); \
+	ddtp_hi = riscv_iommu_readl((iommu), RISCV_IOMMU_REG_DDTP + 4); \
+	ddtp = ((u64)ddtp_hi << 32) | ddtp_lo; \
 	ddtp; })
 
 static int riscv_iommu_iodir_alloc(struct riscv_iommu_device *iommu)
@@ -1501,7 +1504,7 @@ static int riscv_iommu_init_check(struct riscv_iommu_device *iommu)
 	 * regular boot flow and disable translation when we boot into a kexec
 	 * kernel and the previous kernel left them enabled.
 	 */
-	ddtp = riscv_iommu_readq(iommu, RISCV_IOMMU_REG_DDTP);
+	ddtp = riscv_iommu_read_ddtp(iommu);
 	if (ddtp & RISCV_IOMMU_DDTP_BUSY)
 		return -EBUSY;
 
diff --git a/drivers/iommu/riscv/iommu.h b/drivers/iommu/riscv/iommu.h
index 46df79dd549..1b03790fbe1 100644
--- a/drivers/iommu/riscv/iommu.h
+++ b/drivers/iommu/riscv/iommu.h
@@ -11,6 +11,7 @@
 #ifndef _RISCV_IOMMU_H_
 #define _RISCV_IOMMU_H_
 
+#include <linux/io-64-nonatomic-hi-lo.h>
 #include <linux/iommu.h>
 #include <linux/types.h>
 #include <linux/iopoll.h>
@@ -70,17 +71,13 @@ void riscv_iommu_disable(struct riscv_iommu_device *iommu);
 	readl_relaxed((iommu)->reg + (addr))
 
 #define riscv_iommu_readq(iommu, addr) \
-	readq_relaxed((iommu)->reg + (addr))
+	hi_lo_readq_relaxed((iommu)->reg + (addr))
 
 #define riscv_iommu_writel(iommu, addr, val) \
 	writel_relaxed((val), (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)
+	hi_lo_writeq_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, \
-- 
2.50.1 (Apple Git-155)