linux-kernel - [PATCH v2 2/4] MIPS: VDSO: Add implementation of clock

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Date:   Wed, 28 Jun 2017 17:55:29 +0200
From:   Aleksandar Markovic <aleksandar.markovic@...rk.com>
To:     linux-mips@...ux-mips.org
Cc:     Goran Ferenc <goran.ferenc@...tec.com>,
        Miodrag Dinic <miodrag.dinic@...tec.com>,
        Aleksandar Markovic <aleksandar.markovic@...tec.com>,
        Douglas Leung <douglas.leung@...tec.com>,
        James Hogan <james.hogan@...tec.com>,
        linux-kernel@...r.kernel.org, Paul Burton <paul.burton@...tec.com>,
        Petar Jovanovic <petar.jovanovic@...tec.com>,
        Raghu Gandham <raghu.gandham@...tec.com>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: [PATCH v2 2/4] MIPS: VDSO: Add implementation of clock_gettime() fallback

From: Goran Ferenc <goran.ferenc@...tec.com>

This patch adds clock_gettime_fallback() function that wraps assembly
invocation of clock_gettime() syscall using __NR_clock_gettime.

This function is used if pure VDSO implementation of clock_gettime()
does not succeed for any reason. For example, it is called if the
clkid parameter of clock_gettime() is not one of the clkids listed
in the switch-case block of the function __vdso_clock_gettime()
(one such case for clkid is CLOCK_BOOTIME).

If syscall invocation via __NR_clock_gettime fails, register a3 will
be set. So, after the syscall, register a3 is tested and the return
value is negated if it's set.

Signed-off-by: Goran Ferenc <goran.ferenc@...tec.com>
Signed-off-by: Miodrag Dinic <miodrag.dinic@...tec.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@...tec.com>
---
 arch/mips/vdso/gettimeofday.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/arch/mips/vdso/gettimeofday.c b/arch/mips/vdso/gettimeofday.c
index fd7d433..5f63375 100644
--- a/arch/mips/vdso/gettimeofday.c
+++ b/arch/mips/vdso/gettimeofday.c
@@ -20,6 +20,24 @@
 #include <asm/unistd.h>
 #include <asm/vdso.h>
 
+static __always_inline long clock_gettime_fallback(clockid_t _clkid,
+					   struct timespec *_ts)
+{
+	register struct timespec *ts asm("a1") = _ts;
+	register clockid_t clkid asm("a0") = _clkid;
+	register long ret asm("v0");
+	register long nr asm("v0") = __NR_clock_gettime;
+	register long error asm("a3");
+
+	asm volatile(
+	"       syscall\n"
+	: "=r" (ret), "=r" (error)
+	: "r" (clkid), "r" (ts), "r" (nr)
+	: "memory");
+
+	return error ? -ret : ret;
+}
+
 static __always_inline int do_realtime_coarse(struct timespec *ts,
 					      const union mips_vdso_data *data)
 {
@@ -207,7 +225,7 @@ int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
 int __vdso_clock_gettime(clockid_t clkid, struct timespec *ts)
 {
 	const union mips_vdso_data *data = get_vdso_data();
-	int ret;
+	int ret = -1;
 
 	switch (clkid) {
 	case CLOCK_REALTIME_COARSE:
@@ -223,10 +241,11 @@ int __vdso_clock_gettime(clockid_t clkid, struct timespec *ts)
 		ret = do_monotonic(ts, data);
 		break;
 	default:
-		ret = -ENOSYS;
 		break;
 	}
 
-	/* If we return -ENOSYS libc should fall back to a syscall. */
+	if (ret)
+		ret = clock_gettime_fallback(clkid, ts);
+
 	return ret;
 }
-- 
2.7.4