Hi Matt, On Sun, 24 Jul 2016 16:50:39 -0700, Matt Ranostay wrote: > On Wed, Jul 20, 2016 at 11:10 PM, Jean Delvare <jdelvare@xxxxxxx> wrote: > > On Fri, 8 Jul 2016 17:22:10 -0700, Matt Ranostay wrote: > >> On Fri, Jul 8, 2016 at 12:56 AM, Jean Delvare <jdelvare@xxxxxxx> wrote: > >> > On jeu., 2016-07-07 at 19:46 -0700, Matt Ranostay wrote: > >> >> Handling the wraparound requires the data->last_update to be set to an > >> >> initial jiffies value. Otherwise you can start in a state where the > >> >> sensor will never request a reading. > >> > > >> > I can't see how. As I read the code, in the worst case, readings can be > >> > blocked for interval_ms (2 seconds maximum.) > >> > >> On 64-bit systems this is never an issue because the jiffies counter > >> will never wrap around. > >> > >> But my system is a 32-bit ARM core, so the the kernel sets the initial > >> value to 0xfffb6c20 so it will wrap around in 5 minutes to find buggy > >> code. > >> > >> So looking at time_after(0xfffb6c20, 0) will return false always till > >> it finally rolls over. > > > > I've always been confused by how jiffies wrapping is handled. So I > > tested it, and you are correct. > > Yeah but of course on 64-bit systems most just use the lower 32-bits > of the jiffies counter. Actually it seems the jiffies are initialized the same on 32-bit and 64-bit systems, so on 64-bit systems you'll get 33-bit values after 5 minutes, and 34-bit values after about 100 days (if HZ=1000.) But you'll never realistically wrap, you are right. > But since that could rollover, and the unlikely event no sensor data > was read for ~57 days you could be in the same state that couldn't get > a data sample .... At HZ=1000 the jiffies wrap after 49 days, IIRC, not 57. Not that it matters ;-) > Not sure if we want to check this edge case.. and also this effects a > few iio drivers (mostly mine :)) if we really care about this. All other hwmon drivers have the same problem. I can't think of a cheap way to solve this problem, nor do I think we need to. If you monitor your system, you are supposed to poll the device on a regular basis, not once every 49 days. -- Jean Delvare SUSE L3 Support -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
