[PATCH v9 05/20] RISC-V: Define indirect CSR access helpers

[Atish Patra]

From: Atish Patra <atishp@xxxxxxxxxxxx>

The indirect CSR requires multiple instructions to read/write CSR.
Add a few helper macros for ease of usage.

These have to be macros rather than functions. csr_read()/csr_write()
stringify their CSR argument into the inline asm template via
__ASM_STR(), so the CSR number must be a literal token; passing it as a
function parameter emits "csrr %0, iregcsr", which the assembler rejects
with "unknown CSR `iregcsr'". The stringification happens in the
preprocessor, before inlining or constant propagation, so it cannot be
worked around by forcing inlining or by only ever passing constants -
gcc 12, gcc 16 and clang 22 all reject it alike. Underneath, csrr/csrw
encode the CSR as a 12-bit immediate and RISC-V has no register-indirect
form, which is also why asm/csr.h keeps every one of its accessors as a
macro.

Signed-off-by: Atish Patra <atishp@xxxxxxxxxxxx>
Reviewed-by: Charlie Jenkins <thecharlesjenkins@xxxxxxxxx>
Tested-by: Charlie Jenkins <thecharlesjenkins@xxxxxxxxx>
Link: https://patch.msgid.link/20260701-counter_delegation-v8-5-7909f863a645@xxxxxxxx
[pjw@xxxxxxxxxx: expand "ind" abbreviation]
Signed-off-by: Paul Walmsley <pjw@xxxxxxxxxx>
---
 arch/riscv/include/asm/csr_indirect.h | 50 +++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/arch/riscv/include/asm/csr_indirect.h b/arch/riscv/include/asm/csr_indirect.h
new file mode 100644
index 000000000000..0f558fac8f5f
--- /dev/null
+++ b/arch/riscv/include/asm/csr_indirect.h
@@ -0,0 +1,50 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_RISCV_CSR_INDIRECT_H
+#define _ASM_RISCV_CSR_INDIRECT_H
+
+#include <linux/irqflags.h>
+
+#include <asm/csr.h>
+
+/*
+ * These have to be macros rather than functions: csr_read()/csr_write()
+ * stringify their CSR argument into the inline asm template via __ASM_STR(),
+ * so the CSR number must be a literal token at preprocessing time. Passing it
+ * as a function parameter emits "csrr %0, iregcsr", which no assembler can
+ * resolve. RISC-V has no register-indirect form of csrr/csrw - the CSR is a
+ * 12-bit immediate - so the sireg CSR selecting the indirect window cannot
+ * itself be a variable.
+ */
+#define csr_indirect_read(iregcsr, iselbase, iseloff) ({		\
+	unsigned long __value = 0;				\
+	unsigned long __flags;					\
+	local_irq_save(__flags);				\
+	csr_write(CSR_ISELECT, (iselbase) + (iseloff));		\
+	__value = csr_read(iregcsr);				\
+	local_irq_restore(__flags);				\
+	__value;						\
+})
+
+#define csr_indirect_write(iregcsr, iselbase, iseloff, value) ({	\
+	unsigned long __flags;					\
+	local_irq_save(__flags);				\
+	csr_write(CSR_ISELECT, (iselbase) + (iseloff));		\
+	csr_write(iregcsr, (value));				\
+	local_irq_restore(__flags);				\
+})
+
+#define csr_indirect_warl(iregcsr, iselbase, iseloff, warl_val) ({	\
+	unsigned long __old_val = 0, __value = 0;		\
+	unsigned long __flags;					\
+	local_irq_save(__flags);				\
+	csr_write(CSR_ISELECT, (iselbase) + (iseloff));		\
+	__old_val = csr_read(iregcsr);				\
+	csr_write(iregcsr, (warl_val));				\
+	__value = csr_read(iregcsr);				\
+	csr_write(iregcsr, __old_val);				\
+	local_irq_restore(__flags);				\
+	__value;						\
+})
+
+#endif

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2.53.0-Meta