4.14-stable review patch. If anyone has any objections, please let me know.
------------------
From: Javier Carrasco javier.carrasco.cruz@gmail.com
commit 54cf39ec16335dadbe1ba008d8e5e98dae3e26f8 upstream.
The HTU21 offers 4 sampling frequencies: 20, 40, 70 and 120, which are associated to an index that is used to select the right measurement resolution and its corresponding measurement time. The current implementation selects the measurement resolution and the temperature measurement time properly, but it does not select the right humidity measurement time in all cases.
In summary, the 40 and 70 humidity measurement times are swapped.
The reason for that is probably the unusual coding for the measurement resolution. According to the datasheet, the bits [7,0] of the "user register" are used as follows to select the bit resolution:
-------------------------------------------------- | Bit 7 | Bit 0 | RH | Temp | Trh (us) | Tt (us) | -------------------------------------------------- | 0 | 0 | 12 | 14 | 16000 | 50000 | -------------------------------------------------- | 0 | 1 | 8 | 12 | 3000 | 13000 | -------------------------------------------------- | 1 | 0 | 10 | 13 | 5000 | 25000 | -------------------------------------------------- | 1 | 1 | 11 | 11 | 8000 | 7000 | -------------------------------------------------- *This table is available in the official datasheet, page 13/21. I have just appended the times provided in the humidity/temperature tables, pages 3/21, 5/21. Note that always a pair of resolutions is selected.
The sampling frequencies [20, 40, 70, 120] are assigned to a linear index [0..3] which is then coded as follows [1]:
Index [7,0] -------------- idx 0 0,0 idx 1 1,0 idx 2 0,1 idx 3 1,1
That is done that way because the temperature measurements are being used as the reference for the sampling frequency (the frequencies and the temperature measurement times are correlated), so increasing the index always reduces the temperature measurement time and its resolution. Therefore, the temperature measurement time array is as simple as [50000, 25000, 13000, 7000]
On the other hand, the humidity resolution cannot follow the same pattern because of the way it is coded in the "user register", where both resolutions are selected at the same time. The humidity measurement time array is the following: [16000, 3000, 5000, 8000], which defines the following assignments:
Index [7,0] Trh ----------------------- idx 0 0,0 16000 -> right, [0,0] selects 12 bits (Trh = 16000) idx 1 1,0 3000 -> wrong! [1,0] selects 10 bits (Trh = 5000) idx 2 0,1 5000 -> wrong! [0,1] selects 8 bits (Trh = 3000) idx 3 1,1 8000 -> right, [1,1] selects 11 bits (Trh = 8000)
The times have been ordered as if idx = 1 -> [0,1] and idx = 2 -> [1,0], which is not the case for the reason explained above.
So a simple modification is required to obtain the right humidity measurement time array, swapping the values in the positions 1 and 2.
The right table should be the following: [16000, 5000, 3000, 8000]
Fix the humidity measurement time array with the right idex/value coding.
[1] The actual code that makes this coding and assigns it to the current value of the "user register" is the following: config_reg &= 0x7E; config_reg |= ((i & 1) << 7) + ((i & 2) >> 1);
Fixes: d574a87cc311 ("Add meas-spec sensors common part") Signed-off-by: Javier Carrasco javier.carrasco.cruz@gmail.com Link: https://lore.kernel.org/r/20231026-topic-htu21_conversion_time-v1-1-bd257dc4... Cc: Stable@vger.kernel.org Signed-off-by: Jonathan Cameron Jonathan.Cameron@huawei.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/iio/common/ms_sensors/ms_sensors_i2c.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/drivers/iio/common/ms_sensors/ms_sensors_i2c.c +++ b/drivers/iio/common/ms_sensors/ms_sensors_i2c.c @@ -16,8 +16,8 @@ /* Conversion times in us */ static const u16 ms_sensors_ht_t_conversion_time[] = { 50000, 25000, 13000, 7000 }; -static const u16 ms_sensors_ht_h_conversion_time[] = { 16000, 3000, - 5000, 8000 }; +static const u16 ms_sensors_ht_h_conversion_time[] = { 16000, 5000, + 3000, 8000 }; static const u16 ms_sensors_tp_conversion_time[] = { 500, 1100, 2100, 4100, 8220, 16440 };