August 22 2015 4:00 PM, "Jonathan Cameron" <jic23@xxxxxxxxxx> wrote:
> On 20/08/15 15:11, Nicola Corna wrote:
>
>> The Si7013/20/21 modules support 2 read modes:
>> * Hold mode, where the device stretches the clock until the end of the
>> measurement
>> * No Hold mode, where the device replies NACK for every I2C call during
>> the measurement
>> Here the No Hold mode is implemented, selectable with the boolean
>> parameter holdmode=N. The No Hold mode is less efficient, since it
>> requires multiple calls to the device, but it can be used as a fallback if
>> the clock stretching is not supported.
>
> Interesting. Strikes me as something that should really be handled via the i2c
> core (and device tree or similar bindings) rather than inside a driver as
> a module parameter. Perhaps info provided to the i2c client driver
> via a check on whether the device supports clock stretching?
>
Reasonable, but we also have to consider that:
* it can happen that the device supports clock stretching but it is bugged
(like the Raspberry Pi)
* with the clock stretching the i2c bus is completely locked until the end of
the measurement (which can take up to 22.8 ms), while with the No Hold mode the
bus is used every 2-6 ms for very short periods (with a i2c clock at 100 KHz,
each call takes 0.1 ms)
In some cases the No Hold mode is preferable, even if the clock stretching is
supported and working.
Nicola Corna
> I'd like input from Jean on this.
>
>> Signed-off-by: Nicola Corna <nicola@xxxxxxxxxx>
>> ---
>> I've tested this on a Raspberry Pi 2, where the clock stretching is
>> currently bugged.
>> drivers/iio/humidity/si7020.c | 55 +++++++++++++++++++++++++++++++++++++------
>> 1 file changed, 48 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/iio/humidity/si7020.c b/drivers/iio/humidity/si7020.c
>> index 62fdbcf..a8bad04 100644
>> --- a/drivers/iio/humidity/si7020.c
>> +++ b/drivers/iio/humidity/si7020.c
>> @@ -2,6 +2,7 @@
>> * si7020.c - Silicon Labs Si7013/20/21 Relative Humidity and Temp Sensors
>> * Copyright (c) 2013,2014 Uplogix, Inc.
>> * David Barksdale <dbarksdale@xxxxxxxxxxx>
>> + * Copyright (c) 2015 Nicola Corna <nicola@xxxxxxxxxx>
>> *
>> * This program is free software; you can redistribute it and/or modify it
>> * under the terms and conditions of the GNU General Public License,
>> @@ -30,16 +31,33 @@
>> #include <linux/module.h>
>> #include <linux/slab.h>
>> #include <linux/sysfs.h>
>> +#include <linux/jiffies.h>
>>
>> #include <linux/iio/iio.h>
>> #include <linux/iio/sysfs.h>
>>
>> /* Measure Relative Humidity, Hold Master Mode */
>> #define SI7020CMD_RH_HOLD 0xE5
>> +/* Measure Relative Humidity, No Hold Master Mode */
>> +#define SI7020CMD_RH_NO_HOLD 0xF5
>> /* Measure Temperature, Hold Master Mode */
>> #define SI7020CMD_TEMP_HOLD 0xE3
>> +/* Measure Temperature, No Hold Master Mode */
>> +#define SI7020CMD_TEMP_NO_HOLD 0xF3
>> /* Software Reset */
>> #define SI7020CMD_RESET 0xFE
>> +/* Relative humidity measurement timeout (us) */
>> +#define SI7020_RH_TIMEOUT 22800
>> +/* Temperature measurement timeout (us) */
>> +#define SI7020_TEMP_TIMEOUT 10800
>> +/* Minimum delay between retries (No Hold Mode) in us */
>> +#define SI7020_NOHOLD_SLEEP_MIN 2000
>> +/* Maximum delay between retries (No Hold Mode) in us */
>> +#define SI7020_NOHOLD_SLEEP_MAX 6000
>> +
>> +static bool holdmode = 1;
>> +module_param(holdmode, bool, 0644);
>> +MODULE_PARM_DESC(holdmode, "Select whether the measurement has to be done with Hold Mode (clock
>> stretching) or No Hold Mode (repeated calls)");
>>
>> static int si7020_read_raw(struct iio_dev *indio_dev,
>> struct iio_chan_spec const *chan, int *val,
>> @@ -47,16 +65,39 @@ static int si7020_read_raw(struct iio_dev *indio_dev,
>> {
>> struct i2c_client **client = iio_priv(indio_dev);
>> int ret;
>> + unsigned char buf[2];
>> + unsigned long start;
>>
>> switch (mask) {
>> case IIO_CHAN_INFO_RAW:
>> - ret = i2c_smbus_read_word_data(*client,
>> - chan->type == IIO_TEMP ?
>> - SI7020CMD_TEMP_HOLD :
>> - SI7020CMD_RH_HOLD);
>> - if (ret < 0)
>> - return ret;
>> - *val = ret >> 2;
>> + if (holdmode) {
>> + ret = i2c_smbus_read_word_data(*client,
>> + chan->type == IIO_TEMP ?
>> + SI7020CMD_TEMP_HOLD :
>> + SI7020CMD_RH_HOLD);
>> + if (ret < 0)
>> + return ret;
>> + *val = ret >> 2;
>> + } else {
>> + ret = i2c_smbus_write_byte(*client,
>> + chan->type == IIO_TEMP ?
>> + SI7020CMD_TEMP_NO_HOLD :
>> + SI7020CMD_RH_NO_HOLD);
>> + if (ret < 0)
>> + return ret;
>> + start = jiffies;
>> + while ((ret = i2c_master_recv(*client, buf, 2)) < 0) {
>> + if (time_after(jiffies, start +
>> + usecs_to_jiffies(
>> + chan->type == IIO_TEMP ?
>> + SI7020_TEMP_TIMEOUT :
>> + SI7020_RH_TIMEOUT)))
>> + return ret;
>> + usleep_range(SI7020_NOHOLD_SLEEP_MIN,
>> + SI7020_NOHOLD_SLEEP_MAX);
>> + }
>> + *val = ((buf[0] << 8) | buf[1]) >> 2;
>> + }
>> /*
>> * Humidity values can sligthly exceed the 0-100%RH
>> * range and should be corrected by software
--
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