MIPS: VDSO: Add implementations of gettimeofday() and clock_gettime()

Add user-mode implementations of gettimeofday() and clock_gettime() to
the VDSO. This is currently usable with 2 clocksources: the CP0 count
register, which is accessible to user-mode via RDHWR on R2 and later
cores, or the MIPS Global Interrupt Controller (GIC) timer, which
provides a "user-mode visible" section containing a mirror of its
counter registers. This section must be mapped into user memory, which
is done below the VDSO data page.

When a supported clocksource is not in use, the VDSO functions will
return -ENOSYS, which causes libc to fall back on the standard syscall
path.

When support for neither of these clocksources is compiled into the
kernel at all, the VDSO still provides clock_gettime(), as the coarse
realtime/monotonic clocks can still be implemented. However,
gettimeofday() is not provided in this case as nothing can be done
without a suitable clocksource. This causes the symbol lookup to fail
in libc and it will then always use the standard syscall path.

This patch includes a workaround for a bug in QEMU which results in
RDHWR on the CP0 count register always returning a constant (incorrect)
value. A fix for this has been submitted, and the workaround can be
removed after the fix has been in stable releases for a reasonable
amount of time.

A simple performance test which calls gettimeofday() 1000 times in a
loop and calculates the average execution time gives the following
results on a Malta + I6400 (running at 20MHz):

 - Syscall:    ~31000 ns
 - VDSO (GIC): ~15000 ns
 - VDSO (CP0): ~9500 ns

[markos.chandras@imgtec.com:
- Minor code re-arrangements in order for mappings to be made
in the order they appear to the process' address space.
- Move do_{monotonic, realtime} outside of the MIPS_CLOCK_VSYSCALL ifdef
- Use gic_get_usm_range so we can do the GIC mapping in the
arch/mips/kernel/vdso instead of the GIC irqchip driver]

Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11338/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

3 files changed, 246 insertions, 0 deletions

diff --git a/arch/mips/vdso/gettimeofday.c b/arch/mips/vdso/gettimeofday.c
new file mode 100644
index 0000000..ce89c9e
--- /dev/null
+++ b/arch/mips/vdso/gettimeofday.c
@@ -0,0 +1,232 @@
+/*
+ * Copyright (C) 2015 Imagination Technologies
+ * Author: Alex Smith <alex.smith@imgtec.com>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation;  either version 2 of the  License, or (at your
+ * option) any later version.
+ */
+
+#include "vdso.h"
+
+#include <linux/compiler.h>
+#include <linux/irqchip/mips-gic.h>
+#include <linux/time.h>
+
+#include <asm/clocksource.h>
+#include <asm/io.h>
+#include <asm/mips-cm.h>
+#include <asm/unistd.h>
+#include <asm/vdso.h>
+
+static __always_inline int do_realtime_coarse(struct timespec *ts,
+					      const union mips_vdso_data *data)
+{
+	u32 start_seq;
+
+	do {
+		start_seq = vdso_data_read_begin(data);
+
+		ts->tv_sec = data->xtime_sec;
+		ts->tv_nsec = data->xtime_nsec >> data->cs_shift;
+	} while (vdso_data_read_retry(data, start_seq));
+
+	return 0;
+}
+
+static __always_inline int do_monotonic_coarse(struct timespec *ts,
+					       const union mips_vdso_data *data)
+{
+	u32 start_seq;
+	u32 to_mono_sec;
+	u32 to_mono_nsec;
+
+	do {
+		start_seq = vdso_data_read_begin(data);
+
+		ts->tv_sec = data->xtime_sec;
+		ts->tv_nsec = data->xtime_nsec >> data->cs_shift;
+
+		to_mono_sec = data->wall_to_mono_sec;
+		to_mono_nsec = data->wall_to_mono_nsec;
+	} while (vdso_data_read_retry(data, start_seq));
+
+	ts->tv_sec += to_mono_sec;
+	timespec_add_ns(ts, to_mono_nsec);
+
+	return 0;
+}
+
+#ifdef CONFIG_CSRC_R4K
+
+static __always_inline u64 read_r4k_count(void)
+{
+	unsigned int count;
+
+	__asm__ __volatile__(
+	"	.set push\n"
+	"	.set mips32r2\n"
+	"	rdhwr	%0, $2\n"
+	"	.set pop\n"
+	: "=r" (count));
+
+	return count;
+}
+
+#endif
+
+#ifdef CONFIG_CLKSRC_MIPS_GIC
+
+static __always_inline u64 read_gic_count(const union mips_vdso_data *data)
+{
+	void __iomem *gic = get_gic(data);
+	u32 hi, hi2, lo;
+
+	do {
+		hi = __raw_readl(gic + GIC_UMV_SH_COUNTER_63_32_OFS);
+		lo = __raw_readl(gic + GIC_UMV_SH_COUNTER_31_00_OFS);
+		hi2 = __raw_readl(gic + GIC_UMV_SH_COUNTER_63_32_OFS);
+	} while (hi2 != hi);
+
+	return (((u64)hi) << 32) + lo;
+}
+
+#endif
+
+static __always_inline u64 get_ns(const union mips_vdso_data *data)
+{
+	u64 cycle_now, delta, nsec;
+
+	switch (data->clock_mode) {
+#ifdef CONFIG_CSRC_R4K
+	case VDSO_CLOCK_R4K:
+		cycle_now = read_r4k_count();
+		break;
+#endif
+#ifdef CONFIG_CLKSRC_MIPS_GIC
+	case VDSO_CLOCK_GIC:
+		cycle_now = read_gic_count(data);
+		break;
+#endif
+	default:
+		return 0;
+	}
+
+	delta = (cycle_now - data->cs_cycle_last) & data->cs_mask;
+
+	nsec = (delta * data->cs_mult) + data->xtime_nsec;
+	nsec >>= data->cs_shift;
+
+	return nsec;
+}
+
+static __always_inline int do_realtime(struct timespec *ts,
+				       const union mips_vdso_data *data)
+{
+	u32 start_seq;
+	u64 ns;
+
+	do {
+		start_seq = vdso_data_read_begin(data);
+
+		if (data->clock_mode == VDSO_CLOCK_NONE)
+			return -ENOSYS;
+
+		ts->tv_sec = data->xtime_sec;
+		ns = get_ns(data);
+	} while (vdso_data_read_retry(data, start_seq));
+
+	ts->tv_nsec = 0;
+	timespec_add_ns(ts, ns);
+
+	return 0;
+}
+
+static __always_inline int do_monotonic(struct timespec *ts,
+					const union mips_vdso_data *data)
+{
+	u32 start_seq;
+	u64 ns;
+	u32 to_mono_sec;
+	u32 to_mono_nsec;
+
+	do {
+		start_seq = vdso_data_read_begin(data);
+
+		if (data->clock_mode == VDSO_CLOCK_NONE)
+			return -ENOSYS;
+
+		ts->tv_sec = data->xtime_sec;
+		ns = get_ns(data);
+
+		to_mono_sec = data->wall_to_mono_sec;
+		to_mono_nsec = data->wall_to_mono_nsec;
+	} while (vdso_data_read_retry(data, start_seq));
+
+	ts->tv_sec += to_mono_sec;
+	ts->tv_nsec = 0;
+	timespec_add_ns(ts, ns + to_mono_nsec);
+
+	return 0;
+}
+
+#ifdef CONFIG_MIPS_CLOCK_VSYSCALL
+
+/*
+ * This is behind the ifdef so that we don't provide the symbol when there's no
+ * possibility of there being a usable clocksource, because there's nothing we
+ * can do without it. When libc fails the symbol lookup it should fall back on
+ * the standard syscall path.
+ */
+int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
+{
+	const union mips_vdso_data *data = get_vdso_data();
+	struct timespec ts;
+	int ret;
+
+	ret = do_realtime(&ts, data);
+	if (ret)
+		return ret;
+
+	if (tv) {
+		tv->tv_sec = ts.tv_sec;
+		tv->tv_usec = ts.tv_nsec / 1000;
+	}
+
+	if (tz) {
+		tz->tz_minuteswest = data->tz_minuteswest;
+		tz->tz_dsttime = data->tz_dsttime;
+	}
+
+	return 0;
+}
+
+#endif /* CONFIG_CLKSRC_MIPS_GIC */
+
+int __vdso_clock_gettime(clockid_t clkid, struct timespec *ts)
+{
+	const union mips_vdso_data *data = get_vdso_data();
+	int ret;
+
+	switch (clkid) {
+	case CLOCK_REALTIME_COARSE:
+		ret = do_realtime_coarse(ts, data);
+		break;
+	case CLOCK_MONOTONIC_COARSE:
+		ret = do_monotonic_coarse(ts, data);
+		break;
+	case CLOCK_REALTIME:
+		ret = do_realtime(ts, data);
+		break;
+	case CLOCK_MONOTONIC:
+		ret = do_monotonic(ts, data);
+		break;
+	default:
+		ret = -ENOSYS;
+		break;
+	}
+
+	/* If we return -ENOSYS libc should fall back to a syscall. */
+	return ret;
+}
diff --git a/arch/mips/vdso/vdso.h b/arch/mips/vdso/vdso.h
index 0bb6b1a..cfb1be4 100644
--- a/arch/mips/vdso/vdso.h
+++ b/arch/mips/vdso/vdso.h
@@ -77,4 +77,13 @@ static inline const union mips_vdso_data *get_vdso_data(void)
 	return (const union mips_vdso_data *)(get_vdso_base() - PAGE_SIZE);
 }
 
+#ifdef CONFIG_CLKSRC_MIPS_GIC
+
+static inline void __iomem *get_gic(const union mips_vdso_data *data)
+{
+	return (void __iomem *)data - PAGE_SIZE;
+}
+
+#endif /* CONFIG_CLKSRC_MIPS_GIC */
+
 #endif /* __ASSEMBLY__ */
diff --git a/arch/mips/vdso/vdso.lds.S b/arch/mips/vdso/vdso.lds.S
index 21655b6..8df7dd5 100644
--- a/arch/mips/vdso/vdso.lds.S
+++ b/arch/mips/vdso/vdso.lds.S
@@ -95,6 +95,11 @@ PHDRS
 VERSION
 {
 	LINUX_2.6 {
+#ifndef DISABLE_MIPS_VDSO
+	global:
+		__vdso_clock_gettime;
+		__vdso_gettimeofday;
+#endif
 	local: *;
 	};
 }