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

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Date:   Mon, 23 Jul 2018 14:38:58 +0200
From:   Stefan Richter <stefanr@...6.in-berlin.de>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     y2038@...ts.linaro.org, Clemens Ladisch <clemens@...isch.de>,
        Ingo Molnar <mingo@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Mark Rutland <mark.rutland@....com>,
        Hector Martin <marcan@...can.st>,
        linux1394-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RESEND] firewire: ohci: stop using get_seconds() for
 BUS_TIME

On Jul 11 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.
> 
> Reviewed-by: Clemens Ladisch <clemens@...isch.de>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> I notice that Stefan Richter has not been active on the mailing lists
> since February 2018.

Thanks Arnd and Clemens.

I resurrected and updated one of my FireWire enabled PCs, and try to get
pack to reasonable response times to firewire driver patches.

The switch from CLOCK_REALTIME to CLOCK_MONOTONIC looks good to me, but
I'll have another look at the context.

> Andrew, could you pick it up in the meantime?
> ---
>  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;
>  	}

-- 
Stefan Richter
-======---=- -=== =-===
http://arcgraph.de/sr/