A handful drivers want to print a content of the struct timespec64
in a format of %lld:%09ld. In order to make their lives easier, add
the respecting specifier directly to the printf() implementation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 Documentation/core-api/printk-formats.rst | 11 ++++++++--
 lib/tests/printf_kunit.c                  |  4 ++++
 lib/vsprintf.c                            | 25 +++++++++++++++++++++++
 3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
index 7f2f11b48286..c0b1b6089307 100644
--- a/Documentation/core-api/printk-formats.rst
+++ b/Documentation/core-api/printk-formats.rst

@@ -547,11 +547,13 @@ Time and date
 	%pt[RT]s		YYYY-mm-dd HH:MM:SS
 	%pt[RT]d		YYYY-mm-dd
 	%pt[RT]t		HH:MM:SS
-	%pt[RT][dt][r][s]
+	%ptSp			<seconds>.<nanoseconds>

+	%pt[RST][dt][r][s]
 
 For printing date and time as represented by::
 
-	R  struct rtc_time structure
+	R  content of struct rtc_time
+	S  content of struct timespec64
 	T  time64_t type
 
 in human readable format.

@@ -563,6 +565,11 @@ The %pt[RT]s (space) will override ISO 8601 separator by using ' ' (space)
 instead of 'T' (Capital T) between date and time. It won't have any effect
 when date or time is omitted.
 
+The %ptSp is equivalent to %lld.%09ld for the content of the struct timespec64.
+When the other specifiers are given, it becomes the respective equivalent of
+%ptT[dt][r][s].%09ld. In other words, the seconds are being printed in
+the human readable format followed by a dot and nanoseconds.
+
 Passed by reference.
 
 struct clk

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 3f99834fd788..fdd06e8957a3 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c

@@ -2464,6 +2488,7 @@ early_param("no_hash_pointers", no_hash_pointers_enable);
  * - 'g' For block_device name (gendisk + partition number)
  * - 't[RT][dt][r][s]' For time and date as represented by: