From d42914c335fd6031e43309a32d6f4b5b0238fb7c Mon Sep 17 00:00:00 2001 From: Miroslav Lichvar Date: Wed, 29 Aug 2012 15:28:42 +0200 Subject: [PATCH] Improve stability of PHC reading in phc2sys. Make five PHC readings in sequence and pick the quickest one. Signed-off-by: Miroslav Lichvar --- phc2sys.c | 35 +++++++++++++++++++++++------------ 1 file changed, 23 insertions(+), 12 deletions(-) diff --git a/phc2sys.c b/phc2sys.c index 24cc8d6..e51c649 100644 --- a/phc2sys.c +++ b/phc2sys.c @@ -41,6 +41,8 @@ #define max_ppb 512000 #define min_ppb -512000 +#define PHC_READINGS 5 + static clockid_t clock_open(char *device) { int fd = open(device, O_RDWR); @@ -87,25 +89,34 @@ static void clock_step(clockid_t clkid, int64_t ns) static int read_phc(clockid_t clkid, clockid_t sysclk, int rdelay, int64_t *offset, uint64_t *ts) { - struct timespec tsrc, tdst; + struct timespec tdst1, tdst2, tsrc; + int i; + int64_t interval, best_interval = INT64_MAX; if (clkid == CLOCK_INVALID) { return 0; } - if (clock_gettime(clkid, &tsrc)) { - perror("clock_gettime"); - return 0; - } + /* Pick the quickest clkid reading. */ + for (i = 0; i < PHC_READINGS; i++) { + if (clock_gettime(sysclk, &tdst1) || + clock_gettime(clkid, &tsrc) || + clock_gettime(sysclk, &tdst2)) { + perror("clock_gettime"); + return 0; + } - if (clock_gettime(sysclk, &tdst)) { - perror("clock_gettime"); - return 0; - } + interval = (tdst2.tv_sec - tdst1.tv_sec) * NS_PER_SEC + + tdst2.tv_nsec - tdst1.tv_nsec; - *offset = tdst.tv_sec * NS_PER_SEC - tsrc.tv_sec * NS_PER_SEC + - tdst.tv_nsec - tsrc.tv_nsec - rdelay; - *ts = tdst.tv_sec * NS_PER_SEC + tdst.tv_nsec; + if (best_interval > interval) { + best_interval = interval; + *offset = (tdst1.tv_sec - tsrc.tv_sec) * NS_PER_SEC + + tdst1.tv_nsec - tsrc.tv_nsec + + interval / 2 - rdelay; + *ts = tdst2.tv_sec * NS_PER_SEC + tdst2.tv_nsec; + } + } return 1; }