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. > >> 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? -- Jean Delvare SUSE L3 Support -- To unsubscribe from this list: send the line "unsubscribe linux-hwmon" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
