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From:   Arnd Bergmann <arnd@arndb.de>
To:     Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc:     y2038@lists.linaro.org, Arnd Bergmann <arnd@arndb.de>,
        Hector Martin <marcan@marcan.st>,
        Mark Rutland <mark.rutland@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux1394-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org
Subject: [PATCH] firewire: ohci: stop using get_seconds() for BUS_TIME
Date:   Mon, 18 Jun 2018 16:19:24 +0200
Message-Id: <20180618141933.3404739-1-arnd@arndb.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

Signed-off-by: Arnd Bergmann <arnd@arndb.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 45c048751f3b..5125841ea338 100644
--- a/drivers/firewire/ohci.c
+++ b/drivers/firewire/ohci.c
@@ -1765,7 +1765,7 @@ 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;
 	}
-- 
2.9.0