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

Message ID 20191108213257.3097633-17-arnd@arndb.de
State Accepted
Commit b98c7518c5345ac5f930fd40ce9d8d2b8dc2ba06
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Series
  • [01/16] staging: exfat: use prandom_u32() for i_generation
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Commit Message

Arnd Bergmann Nov. 8, 2019, 9:32 p.m.
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>

---
 drivers/firewire/ohci.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.20.0

Comments

Stefan Richter Nov. 13, 2019, 8:04 p.m. | #1
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
-======---== =-== -==-=
http://arcgraph.de/sr/

Patch

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;
 	}