Re: [PATCH 16/16] firewire: ohci: stop using get_seconds() for BUS_TIME

Stefan Richter
2019-11-13T20:05:09Z

On Nov 08 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@ladisch.de>
> Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
> Link: https://lore.kernel.org/lkml/20180711124923.1205200-1-arnd@arndb.de/
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Committed to linux1394.git:for-next.
Thank you for the patch and your extraordinary patience.

> ---
> 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;
> }

--
Stefan Richter
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