[PATCH v2] thermal: rockchip: make temperature reporting much more accurate

[Daniel Kurtz]

Hi Caesar,

Some drive-by review comments inline...

On Mon, Jan 19, 2015 at 7:55 AM, Caesar Wang <wxt at rock-chips.com> wrote:
> In general, the kernel should report temperature readings exactly as
> reported by the hardware. The cpu / gpu thermal driver works in 5 degree
> increments,but we ought to do more accurate. The temperature will do
> linear interpolation between the entries in the table.
>
> Test= $md5sum /dev/zero &
> $while true; do grep "" /sys/class/thermal/thermal_zone[1-2]/temp;
> sleep .5; done
>
> e.g. We can get the result as follows:
>     /sys/class/thermal/thermal_zone1/temp:39994
>     /sys/class/thermal/thermal_zone2/temp:39086
>     /sys/class/thermal/thermal_zone1/temp:39994
>     /sys/class/thermal/thermal_zone2/temp:39540
>     /sys/class/thermal/thermal_zone1/temp:39540
>     /sys/class/thermal/thermal_zone2/temp:39540
>     /sys/class/thermal/thermal_zone1/temp:39540
>     /sys/class/thermal/thermal_zone2/temp:39994
>
> Signed-off-by: Caesar Wang <wxt at rock-chips.com>
> Reviewed-by: Dmitry Torokhov <dmitry.torokhov at gmail.com>
>
> ---
>
> Changes in v2:
> Reviewed-by: Dmitry Torokhov <dmitry.torokhov at gmail.com>
>
>  drivers/thermal/rockchip_thermal.c | 26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/thermal/rockchip_thermal.c b/drivers/thermal/rockchip_thermal.c
> index 1bcddfc..a7ae23a 100644
> --- a/drivers/thermal/rockchip_thermal.c
> +++ b/drivers/thermal/rockchip_thermal.c
> @@ -193,19 +193,18 @@ static u32 rk_tsadcv2_temp_to_code(long temp)
>
>  static long rk_tsadcv2_code_to_temp(u32 code)
>  {
> -       int high, low, mid;
> -
> -       low = 0;
> -       high = ARRAY_SIZE(v2_code_table) - 1;
> -       mid = (high + low) / 2;
> +       unsigned int low = 0;
> +       unsigned int high = ARRAY_SIZE(v2_code_table) - 1;
> +       unsigned int mid = (low + high) / 2;
> +       unsigned int scale;
>
>         if (code > v2_code_table[low].code || code < v2_code_table[high].code)
>                 return 125000; /* No code available, return max temperature */

I think if the temp sensor reading was invalid we should return an
error code rather than just silently returning "max temp".
Returning an error here could then be propagated up by its only
caller, the ->get_temp() callback.

Note: 'code < v2_code_table[high].code' is always false, since code is
unsigned, and the last entry is 0.

Also, the check above doesn't reject "code == 0xfff".
Even worse, I believe in that case the below algorithm will eventually
set mid=0 and access v2_code_table[-1].code.

>
>         while (low <= high) {
> -               if (code >= v2_code_table[mid].code && code <
> -                   v2_code_table[mid - 1].code)
> -                       return v2_code_table[mid].temp;
> +               if (code >= v2_code_table[mid].code &&
> +                   code < v2_code_table[mid - 1].code)
> +                       break;
>                 else if (code < v2_code_table[mid].code)
>                         low = mid + 1;
>                 else
> @@ -213,7 +212,16 @@ static long rk_tsadcv2_code_to_temp(u32 code)
>                 mid = (low + high) / 2;
>         }
>
> -       return 125000;
> +       /*
> +        * The 5C granularity provided by the table is too much. Let's
> +        * assume that the relationship between sensor readings and
> +        * temperature between 2 table entries is linear and extrapolate

I think this is "interpolate", not "extrapolate".

> +        * to produce less granular result.
> +        */
> +       scale = (v2_code_table[mid].temp - v2_code_table[mid - 1].temp) /
> +               (v2_code_table[mid - 1].code - v2_code_table[mid].code);
> +       return v2_code_table[mid - 1].temp +
> +                       (v2_code_table[mid - 1].code - code) * scale;

Assuming the product fits in an unsigned long (and it will, since code
is only 12-bits), it is more precise to multiply first and then
divide, something like this:

   num = v2_code_table[mid].temp - v2_code_table[mid - 1].temp;
   num *= v2_code_table[mid - 1].code - code;
   denom = v2_code_table[mid - 1].code - v2_code_table[mid].code;
   return v2_code_table[mid - 1].temp + (num / denom);

-Daniel

>  }
>
>  /**
> --
> 1.9.1
>
>
>
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