[PATCH] RISC-V: Resurrect the MMIO timer implementation for M-mode systems

[Palmer Dabbelt]

The K210 doesn't implement rdtime in M-mode, and since that's where Linux runs
in the NOMMU systems that means we can't use rdtime.  The K210 is the only
system that anyone is currently running NOMMU or M-mode on, so here we're just
inlining the timer read directly.

Signed-off-by: Palmer Dabbelt <palmerdabbelt at google.com>
---
I don't actually have a K210 so I haven't tested this.  If nobody else has the
time to I'll put together a QEMU that doesn't support rdtime in M-mode, but
I've yet to mess around with the !MMU stuff so that might take a little while.
This certainly doesn't seem worse than what's there right now, though, as
rdtime isn't valid in M-mode on the K210 (our only M-mode platform).
---
 arch/riscv/include/asm/clint.h    | 26 ++++++++++++++++++++++++++
 arch/riscv/include/asm/timex.h    | 27 +++++++++++++++++++++++++++
 drivers/clocksource/timer-clint.c | 17 +++++++++++++++++
 3 files changed, 70 insertions(+)
 create mode 100644 arch/riscv/include/asm/clint.h

diff --git a/arch/riscv/include/asm/clint.h b/arch/riscv/include/asm/clint.h
new file mode 100644
index 000000000000..0789fd37b40a
--- /dev/null
+++ b/arch/riscv/include/asm/clint.h
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2020 Google, Inc
+ */
+
+#ifndef _ASM_RISCV_CLINT_H
+#define _ASM_RISCV_CLINT_H
+
+#include <linux/types.h>
+#include <asm/mmio.h>
+
+#ifdef CONFIG_RISCV_M_MODE
+/*
+ * This lives in the CLINT driver, but is accessed directly by timex.h to avoid
+ * any overhead when accessing the MMIO timer.
+ *
+ * The ISA defines mtime as a 64-bit memory-mapped register that increments at
+ * a constant frequency, but it doesn't define some other constraints we depend
+ * on (most notably ordering constraints, but also some simpler stuff like the
+ * memory layout).  Thus, this is called "clint_time_val" instead of something
+ * like "riscv_mtime", to signify that these non-ISA assumptions must hold.
+ */
+extern u64 __iomem *clint_time_val;
+#endif
+
+#endif
diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
index a3fb85d505d4..7f659dda0032 100644
--- a/arch/riscv/include/asm/timex.h
+++ b/arch/riscv/include/asm/timex.h
@@ -10,6 +10,31 @@
 
 typedef unsigned long cycles_t;
 
+#ifdef CONFIG_RISCV_M_MODE
+
+#include <asm/clint.h>
+
+#ifdef CONFIG_64BIT
+static inline cycles_t get_cycles(void)
+{
+	return readq_relaxed(clint_time_val);
+}
+#else /* !CONFIG_64BIT */
+static inline u32 get_cycles(void)
+{
+	return readl_relaxed(((u32 *)clint_time_val));
+}
+#define get_cycles get_cycles
+
+static inline u32 get_cycles_hi(void)
+{
+	return readl_relaxed(((u32 *)clint_time_val) + 1);
+}
+#define get_cycles_hi get_cycles_hi
+#endif /* CONFIG_64BIT */
+
+#else /* CONFIG_RISCV_M_MODE */
+
 static inline cycles_t get_cycles(void)
 {
 	return csr_read(CSR_TIME);
@@ -41,6 +66,8 @@ static inline u64 get_cycles64(void)
 }
 #endif /* CONFIG_64BIT */
 
+#endif /* !CONFIG_RISCV_M_MODE */
+
 #define ARCH_HAS_READ_CURRENT_TIMER
 static inline int read_current_timer(unsigned long *timer_val)
 {
diff --git a/drivers/clocksource/timer-clint.c b/drivers/clocksource/timer-clint.c
index 8eeafa82c03d..d17367dee02c 100644
--- a/drivers/clocksource/timer-clint.c
+++ b/drivers/clocksource/timer-clint.c
@@ -19,6 +19,11 @@
 #include <linux/interrupt.h>
 #include <linux/of_irq.h>
 #include <linux/smp.h>
+#include <linux/timex.h>
+
+#ifndef CONFIG_RISCV_M_MODE
+#include <asm/clint.h>
+#endif
 
 #define CLINT_IPI_OFF		0
 #define CLINT_TIMER_CMP_OFF	0x4000
@@ -31,6 +36,10 @@ static u64 __iomem *clint_timer_val;
 static unsigned long clint_timer_freq;
 static unsigned int clint_timer_irq;
 
+#ifdef CONFIG_RISCV_M_MODE
+u64 __iomem *clint_time_val;
+#endif
+
 static void clint_send_ipi(const struct cpumask *target)
 {
 	unsigned int cpu;
@@ -184,6 +193,14 @@ static int __init clint_timer_init_dt(struct device_node *np)
 	clint_timer_val = base + CLINT_TIMER_VAL_OFF;
 	clint_timer_freq = riscv_timebase;
 
+#ifdef CONFIG_RISCV_M_MODE
+	/*
+	 * Yes, that's an odd naming scheme.  time_val is public, but hopefully
+	 * will die in favor of something cleaner.
+	 */
+	clint_time_val = clint_timer_val;
+#endif
+
 	pr_info("%pOFP: timer running at %ld Hz\n", np, clint_timer_freq);
 
 	rc = clocksource_register_hz(&clint_clocksource, clint_timer_freq);
-- 
2.28.0.618.gf4bc123cb7-goog