Re: [PATCH] iio: common: ms_sensors: ms_sensors_i2c: fix humidity conversion time table

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 26 Oct 2023 17:44:49 +0200
Javier Carrasco <javier.carrasco.cruz@gmail.com> wrote:

> 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>

Good description.  That's horrible :(

Applied to the fixes-togreg branch of iio.git and marked for stable.
Note however that I probably won't send a fixes pull request until after rc1
is out in about 3 weeks time.

Thanks,

Jonathan
> ---
>  drivers/iio/common/ms_sensors/ms_sensors_i2c.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iio/common/ms_sensors/ms_sensors_i2c.c b/drivers/iio/common/ms_sensors/ms_sensors_i2c.c
> index 6633b35a94e6..9c9bc77003c7 100644
> --- a/drivers/iio/common/ms_sensors/ms_sensors_i2c.c
> +++ b/drivers/iio/common/ms_sensors/ms_sensors_i2c.c
> @@ -15,8 +15,8 @@
>  /* 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 };
>  
> 
> ---
> base-commit: 611da07b89fdd53f140d7b33013f255bf0ed8f34
> change-id: 20231026-topic-htu21_conversion_time-9ea81e86703f
> 
> Best regards,