phc: Add a method to query the maximum adjustment.

Signed-off-by: Richard Cochran <[email protected]>

phc.c

@@ -16,11 +16,15 @@
  * with this program; if not, write to the Free Software Foundation, Inc.,
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
+#include <stdio.h>
+#include <sys/ioctl.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <unistd.h>
 
+#include <linux/ptp_clock.h>
+
 #include "phc.h"
 
 clockid_t phc_open(char *phc)
@@ -37,3 +41,16 @@ void phc_close(clockid_t clkid)
 
 	close(CLOCKID_TO_FD(clkid));
 }
+
+int phc_max_adj(clockid_t clkid)
+{
+	int fd = CLOCKID_TO_FD(clkid);
+	struct ptp_clock_caps caps;
+
+	if (ioctl(fd, PTP_CLOCK_GETCAPS, &caps)) {
+		perror("PTP_CLOCK_GETCAPS");
+		return 0;
+	}
+
+	return caps.max_adj;
+}

phc.h

@@ -37,4 +37,13 @@ clockid_t phc_open(char *phc);
  */
 void phc_close(clockid_t clkid);
 
+/**
+ * Query the maximum frequency adjustment of a PTP hardware clock device.
+ *
+ * @param clkid A clock ID obtained using phc_open().
+ *
+ * @return The clock's maximum frequency adjustment in parts per billion.
+ */
+int phc_max_adj(clockid_t clkid);
+
 #endif