On Wed, Jul 20, 2016 at 11:10 PM, Jean Delvare <jdelvare@xxxxxxx> wrote: > Hi Matt, > > 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. 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 .... 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. >> >> Cc: Guenter Roeck <linux@xxxxxxxxxxxx> >> >> Cc: David Frey <david.frey@xxxxxxxxxxxxx> >> >> Signed-off-by: Matt Ranostay <mranostay@xxxxxxxxx> >> >> --- >> >> drivers/hwmon/sht3x.c | 2 +- >> >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> >> >> diff --git a/drivers/hwmon/sht3x.c b/drivers/hwmon/sht3x.c >> >> index 450645b6053d..05a925257938 100644 >> >> --- a/drivers/hwmon/sht3x.c >> >> +++ b/drivers/hwmon/sht3x.c >> >> @@ -722,7 +722,7 @@ static int sht3x_probe(struct i2c_client *client, >> >> data->setup.blocking_io = false; >> >> data->setup.high_precision = true; >> >> data->mode = 0; >> >> - data->last_update = 0; >> >> + data->last_update = jiffies; >> >> data->client = client; >> >> crc8_populate_msb(sht3x_crc8_table, SHT3X_CRC8_POLYNOMIAL); >> >> >> > >> > Both look equally wrong to me. With your proposal, accessing the sysfs >> > attributes right after loading the driver will not trigger a reading. >> > >> > In order to guarantee that the first access will trigger a reading, >> > data->last_update should be initialized to jiffies - >> > msecs_to_jiffies(2000) (the maximum interval value.) >> >> Ok that is fine. Rather do jiffies + (2 * HZ) > > 2 * HZ is fine. But it's minus, not plus. You want to initialize the > last update time to 2 seconds BEFORE the driver is being loaded, so > that the current time is at least 2 seconds AFTER the (fake) last > update at first access. And you should subtract another jiffy to be on > the safe side. > > Can you please send an updated patch? Sure will do. > > -- > 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
