Linux/ADS7843: work incorrect ,get event (get value x:0 y:4095 Rt:127 )

Tool/software: Linux

linux4.4 

driver:drivers/input/touchscreen/ads7846.c

dts:

ads7843: ads7843@00{
status = "okay";
compatible = "ti,ads7843";
reg = <0x00>;
interrupt-parent = <&gpio3>;
interrupts = <1 0>; //
spi-max-frequency = <1000000>;
pendown-gpio = <&gpio3 1 0>; //GPIO3_A1
//vcc-supply = <&reg_vcc3>;

ti,x-min = /bits/ 16 <0>;
ti,x-max = /bits/ 16 <8000>;
ti,y-min = /bits/ 16 <0>;
ti,y-max = /bits/ 16 <4800>;
ti,x-plate-ohms = /bits/ 16 <40>;
ti,pressure-max = /bits/ 16 <255>;

wakeup-source;

};

func:

ads7846_report_state

if (Rt) {
struct input_dev *input = ts->input;

if (ts->swap_xy)
swap(x, y);

if (!ts->pendown) {
input_report_key(input, BTN_TOUCH, 1);
ts->pendown = true;
dev_vdbg(&ts->spi->dev, "DOWN\n");
}

input_report_abs(input, ABS_X, x);
input_report_abs(input, ABS_Y, y);
input_report_abs(input, ABS_PRESSURE, ts->pressure_max - Rt);

input_sync(input);
dev_vdbg(&ts->spi->dev, "%4d/%4d/%4d\n", x, y, Rt);
printk("lintest ads7846_report_state %4d/%4d/%4d\n", x, y, Rt);
}

when touch the panel 

print:  lintest ads7846_report_state 0 4095 127

i don't know why...