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

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Date:   Tue, 19 Jun 2018 08:26:43 +0200
From:   Clemens Ladisch <clemens@...isch.de>
To:     Arnd Bergmann <arnd@...db.de>,
        Stefan Richter <stefanr@...6.in-berlin.de>
Cc:     Mark Rutland <mark.rutland@....com>,
        linux1394-devel@...ts.sourceforge.net, y2038@...ts.linaro.org,
        Hector Martin <marcan@...can.st>, linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] firewire: ohci: stop using get_seconds() for BUS_TIME

Arnd Bergmann wrote:
> 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@...db.de>

Reviewed-By: Clemens Ladisch <clemens@...isch.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;
>  	}