* [PATCH] hwmon: sht3x: set initial jiffies to last_update
@ 2016-07-08 2:46 Matt Ranostay
2016-07-08 7:56 ` Jean Delvare
0 siblings, 1 reply; 6+ messages in thread
From: Matt Ranostay @ 2016-07-08 2:46 UTC (permalink / raw
To: linux-hwmon; +Cc: Matt Ranostay, Guenter Roeck, David Frey
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.
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: David Frey <david.frey@sensirion.com>
Signed-off-by: Matt Ranostay <mranostay@gmail.com>
---
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);
--
2.7.4
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH] hwmon: sht3x: set initial jiffies to last_update
2016-07-08 2:46 [PATCH] hwmon: sht3x: set initial jiffies to last_update Matt Ranostay
@ 2016-07-08 7:56 ` Jean Delvare
2016-07-09 0:22 ` Matt Ranostay
0 siblings, 1 reply; 6+ messages in thread
From: Jean Delvare @ 2016-07-08 7:56 UTC (permalink / raw
To: Matt Ranostay; +Cc: linux-hwmon, Guenter Roeck, David Frey
Hi Matt,
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.)
>
> Cc: Guenter Roeck <linux@roeck-us.net>
> Cc: David Frey <david.frey@sensirion.com>
> Signed-off-by: Matt Ranostay <mranostay@gmail.com>
> ---
> 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.)
--
Jean Delvare
SUSE L3 Support
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] hwmon: sht3x: set initial jiffies to last_update
2016-07-08 7:56 ` Jean Delvare
@ 2016-07-09 0:22 ` Matt Ranostay
2016-07-21 6:10 ` Jean Delvare
0 siblings, 1 reply; 6+ messages in thread
From: Matt Ranostay @ 2016-07-09 0:22 UTC (permalink / raw
To: Jean Delvare; +Cc: linux-hwmon, Guenter Roeck, David Frey
On Fri, Jul 8, 2016 at 12:56 AM, Jean Delvare <jdelvare@suse.de> wrote:
> Hi Matt,
>
> 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.
>
>>
>> Cc: Guenter Roeck <linux@roeck-us.net>
>> Cc: David Frey <david.frey@sensirion.com>
>> Signed-off-by: Matt Ranostay <mranostay@gmail.com>
>> ---
>> 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)
>
> --
> Jean Delvare
> SUSE L3 Support
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] hwmon: sht3x: set initial jiffies to last_update
2016-07-09 0:22 ` Matt Ranostay
@ 2016-07-21 6:10 ` Jean Delvare
2016-07-24 23:50 ` Matt Ranostay
0 siblings, 1 reply; 6+ messages in thread
From: Jean Delvare @ 2016-07-21 6:10 UTC (permalink / raw
To: Matt Ranostay; +Cc: linux-hwmon, Guenter Roeck, David Frey
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@suse.de> 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@roeck-us.net>
> >> Cc: David Frey <david.frey@sensirion.com>
> >> Signed-off-by: Matt Ranostay <mranostay@gmail.com>
> >> ---
> >> 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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] hwmon: sht3x: set initial jiffies to last_update
2016-07-21 6:10 ` Jean Delvare
@ 2016-07-24 23:50 ` Matt Ranostay
2016-07-25 8:42 ` Jean Delvare
0 siblings, 1 reply; 6+ messages in thread
From: Matt Ranostay @ 2016-07-24 23:50 UTC (permalink / raw
To: Jean Delvare, linux-iio@vger.kernel.org, Jonathan Cameron
Cc: linux-hwmon, Guenter Roeck, David Frey
On Wed, Jul 20, 2016 at 11:10 PM, Jean Delvare <jdelvare@suse.de> 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@suse.de> 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@roeck-us.net>
>> >> Cc: David Frey <david.frey@sensirion.com>
>> >> Signed-off-by: Matt Ranostay <mranostay@gmail.com>
>> >> ---
>> >> 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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] hwmon: sht3x: set initial jiffies to last_update
2016-07-24 23:50 ` Matt Ranostay
@ 2016-07-25 8:42 ` Jean Delvare
0 siblings, 0 replies; 6+ messages in thread
From: Jean Delvare @ 2016-07-25 8:42 UTC (permalink / raw
To: Matt Ranostay
Cc: linux-iio, Jonathan Cameron, linux-hwmon, Guenter Roeck,
David Frey
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@suse.de> 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@suse.de> 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
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2016-07-25 8:42 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
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2016-07-08 2:46 [PATCH] hwmon: sht3x: set initial jiffies to last_update Matt Ranostay
2016-07-08 7:56 ` Jean Delvare
2016-07-09 0:22 ` Matt Ranostay
2016-07-21 6:10 ` Jean Delvare
2016-07-24 23:50 ` Matt Ranostay
2016-07-25 8:42 ` Jean Delvare
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