[PATCH v2] input: wacom: Report correct touch contact size for I5/Bamboo

This changes how the touch packets for Intuos5 and 3rd-gen Bamboo
are interpreted, so that proper values for the MAJOR and MINOR
axes are reported. Instead of using the amplitude field (data[6]),
we use the size field (data[5]) and do some calculation to
transform it from a scaled-down area into axis lengths.

Note that even though we assume a circular contact, both MAJOR and
MINOR are reported since the resolution of the X and Y axes differ.
---
 Changes from v1:
   * Changed name from "Use touch size for ABS_MT_TOUCH_MAJOR"

   * Also populate ABS_MT_TOUCH_MINOR to avoid some userspace
     assumptions.

   * Experimentally determined the calculations necessary to produce
     proper values instead of simply using the raw value in data[5].

 drivers/input/tablet/wacom_wac.c |   28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/drivers/input/tablet/wacom_wac.c b/drivers/input/tablet/wacom_wac.c
index 004bc1b..940caf3 100644
--- a/drivers/input/tablet/wacom_wac.c
+++ b/drivers/input/tablet/wacom_wac.c

@@ -25,6 +25,11 @@
 #define WACOM_INTUOS_RES       100
 #define WACOM_INTUOS3_RES      200

+/* Scale factor relating reported contact size to logical contact area.
+ * 2^14/pi is a good approximation on Intuos5 and 3rd-gen Bamboo
+ */
+#define WACOM_CONTACT_AREA_SCALE 5215
+
 static int wacom_penpartner_irq(struct wacom_wac *wacom)
 {
        unsigned char *data = wacom->data;

@@ -1036,11 +1041,21 @@ static void wacom_bpt3_touch_msg(struct wacom_wac *wacom, unsigned char *data)
        if (touch) {
                int x = (data[2] << 4) | (data[4] >> 4);
                int y = (data[3] << 4) | (data[4] & 0x0f);
-               int w = data[6];
+               int a = data[5];
+
+               // "a" is a scaled-down area which we assume is roughly
+               // circular and which can be described as: a*C=(pi*r^2).
+               int x_res = input_abs_get_res(input, ABS_X);
+               int y_res = input_abs_get_res(input, ABS_Y);
+               int logical_diameter  = 2 * int_sqrt(a * WACOM_CONTACT_AREA_SCALE);
+               int physical_diameter = logical_diameter / ((x_res + y_res) / 2);
+               int logical_width  = physical_diameter * x_res;
+               int logical_height = physical_diameter * y_res;

                input_report_abs(input, ABS_MT_POSITION_X, x);
                input_report_abs(input, ABS_MT_POSITION_Y, y);
-               input_report_abs(input, ABS_MT_TOUCH_MAJOR, w);
+               input_report_abs(input, ABS_MT_TOUCH_MAJOR, logical_width);
+               input_report_abs(input, ABS_MT_TOUCH_MINOR, logical_height);
        }
 }

@@ -1520,7 +1535,9 @@ int wacom_setup_input_capabilities(struct input_dev *input_dev,
                        input_mt_init_slots(input_dev, features->touch_max);

                        input_set_abs_params(input_dev, ABS_MT_TOUCH_MAJOR,
-                                            0, 255, 0, 0);
+                                            0, features->x_max, 0, 0);
+                       input_set_abs_params(input_dev, ABS_MT_TOUCH_MINOR,
+                                            0, features->y_max, 0, 0);

                        input_set_abs_params(input_dev, ABS_MT_POSITION_X,
                                             0, features->x_max,

@@ -1625,7 +1642,10 @@ int wacom_setup_input_capabilities(struct input_dev *input_dev,

                                input_set_abs_params(input_dev,
                                                     ABS_MT_TOUCH_MAJOR,
-                                                    0, 255, 0, 0);
+                                                    0, features->x_max, 0, 0);
+                               input_set_abs_params(input_dev,
+                                                    ABS_MT_TOUCH_MINOR,
+                                                    0, features->y_max, 0, 0);
                        }

                        input_set_abs_params(input_dev, ABS_MT_POSITION_X,
--

1.7.10.2