linux-kernel - Re: [PATCH] iio: common: ssp_sensors: use ktime_get_real

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Date:   Sat, 2 Dec 2017 11:49:49 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     y2038@...ts.linaro.org, Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        <k.wrona@...sung.com>
Subject: Re: [PATCH] iio: common: ssp_sensors: use ktime_get_real_ns()
 timestamps

On Mon, 27 Nov 2017 12:51:48 +0100
Arnd Bergmann <arnd@...db.de> wrote:

> getnstimeofday() suffers from the overflow in y2038 on 32-bit
> architectures and requires a conversion into the nanosecond format that
> we want here.
> 
> This changes ssp_parse_dataframe() to use ktime_get_real_ns() directly,
> which does not have that problem.
> 
> An open question is what time base should be used here. Normally
> timestamps should use ktime_get_ns() or ktime_get_boot_ns() to read
> monotonic time instead of "real" time, which suffers from time jumps
> due to settimeofday() calls or leap seconds.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

+cc Karol Wrona.

Fix looks fine to me, but might be nice to address that and the
question of using a monotonic clock all in one go.

This is probably a copy of an ancient piece of my stupidity
where I used the wrong clock for the core IIO timestamping
code.  We fixed that a while back by letting the user
chose the clock.  If you are running slow enough you actually
do want the 'real' time - probably never the case here though.

If we don't hear from Karol for a few weeks I'll pick this
up and we can address the wrong timestamp sometime in the
future!

Jonathan

> ---
>  drivers/iio/common/ssp_sensors/ssp_spi.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/common/ssp_sensors/ssp_spi.c b/drivers/iio/common/ssp_sensors/ssp_spi.c
> index 704284a475ae..2ab106bb3e03 100644
> --- a/drivers/iio/common/ssp_sensors/ssp_spi.c
> +++ b/drivers/iio/common/ssp_sensors/ssp_spi.c
> @@ -277,12 +277,9 @@ static int ssp_handle_big_data(struct ssp_data *data, char *dataframe, int *idx)
>  static int ssp_parse_dataframe(struct ssp_data *data, char *dataframe, int len)
>  {
>  	int idx, sd;
> -	struct timespec ts;
>  	struct ssp_sensor_data *spd;
>  	struct iio_dev **indio_devs = data->sensor_devs;
>  
> -	getnstimeofday(&ts);
> -
>  	for (idx = 0; idx < len;) {
>  		switch (dataframe[idx++]) {
>  		case SSP_MSG2AP_INST_BYPASS_DATA:
> @@ -329,7 +326,7 @@ static int ssp_parse_dataframe(struct ssp_data *data, char *dataframe, int len)
>  	}
>  
>  	if (data->time_syncing)
> -		data->timestamp = ts.tv_sec * 1000000000ULL + ts.tv_nsec;
> +		data->timestamp = ktime_get_real_ns();
>  
>  	return 0;
>  }