linux-kernel - [PATCH 16/16] firewire: ohci: stop using get_seconds() for BUS

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Date:   Fri,  8 Nov 2019 22:32:54 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     y2038@...ts.linaro.org, Stefan Richter <stefanr@...6.in-berlin.de>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Clemens Ladisch <clemens@...isch.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Richard Fontana <rfontana@...hat.com>,
        linux1394-devel@...ts.sourceforge.net
Subject: [PATCH 16/16] firewire: ohci: stop using get_seconds() for BUS_TIME

The ohci driver uses the get_seconds() function to implement the 32-bit
CSR_BUS_TIME register. This was added in 2010 commit a48777e03ad5
("firewire: add CSR BUS_TIME support").

As get_seconds() returns a 32-bit value (on 32-bit architectures), it
seems like a good fit for that register, but it is also deprecated because
of the y2038/y2106 overflow problem, and should be replaced throughout
the kernel with either ktime_get_real_seconds() or ktime_get_seconds().

I'm using the latter here, which uses monotonic time. This has the
advantage of behaving better during concurrent settimeofday() updates
or leap second adjustments and won't overflow a 32-bit integer, but
the downside of using CLOCK_MONOTONIC instead of CLOCK_REALTIME is
that the observed values are not related to external clocks.

If we instead need UTC but can live with clock jumps or overflows,
then we should use ktime_get_real_seconds() instead, retaining the
existing behavior.

Reviewed-by: Clemens Ladisch <clemens@...isch.de>
Cc: Stefan Richter <stefanr@...6.in-berlin.de>
Link: https://lore.kernel.org/lkml/20180711124923.1205200-1-arnd@arndb.de/
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/firewire/ohci.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firewire/ohci.c b/drivers/firewire/ohci.c
index 522f3addb5bd..33269316f111 100644
--- a/drivers/firewire/ohci.c
+++ b/drivers/firewire/ohci.c
@@ -1752,7 +1752,7 @@ static u32 update_bus_time(struct fw_ohci *ohci)

 	if (unlikely(!ohci->bus_time_running)) {
 		reg_write(ohci, OHCI1394_IntMaskSet, OHCI1394_cycle64Seconds);
-		ohci->bus_time = (lower_32_bits(get_seconds()) & ~0x7f) |
+		ohci->bus_time = (lower_32_bits(ktime_get_seconds()) & ~0x7f) |
 		                 (cycle_time_seconds & 0x40);
 		ohci->bus_time_running = true;
 	}
-- 
2.20.0