thermal: rockchip: make temperature reporting much more accurate

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

Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

1 files changed, 23 insertions, 13 deletions

diff --git a/drivers/thermal/rockchip_thermal.c b/drivers/thermal/rockchip_thermal.c
index 9c6ce548e363..3aa46ac7cdbc 100644
--- a/drivers/thermal/rockchip_thermal.c
+++ b/drivers/thermal/rockchip_thermal.c
@@ -193,19 +193,20 @@ 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;
-
-        if (code > v2_code_table[low].code || code < v2_code_table[high].code)
-                return 125000; /* No code available, return max temperature */
+        unsigned int low = 0;
+        unsigned int high = ARRAY_SIZE(v2_code_table) - 1;
+        unsigned int mid = (low + high) / 2;
+        unsigned int num;
+        unsigned long denom;
+
+        /* Invalid code, return -EAGAIN */
+        if (code > TSADCV2_DATA_MASK)
+                return -EAGAIN;
 
-        while (low <= high) {
-                if (code >= v2_code_table[mid].code && code <
-                    v2_code_table[mid - 1].code)
-                        return v2_code_table[mid].temp;
+        while (low <= high && mid) {
+                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 +214,16 @@ 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 interpolate
+         * to produce less granular result.
+         */
+        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);
 }
 
 /**