Input: add input_copy_abs() function

Add a new helper function to copy absinfo from one input_dev to
another input_dev.

This is useful to e.g. setup a pen/stylus input-device for combined
touchscreen/pen hardware where the pen uses the same coordinates as
the touchscreen.

Suggested-by: Dmitry Torokhov <[e-mail]>
Signed-off-by: Hans de Goede <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/input.c

···
 }
 EXPORT_SYMBOL(input_set_abs_params);
 
+/**
+ * input_copy_abs - Copy absinfo from one input_dev to another
+ * @dst: Destination input device to copy the abs settings to
+ * @dst_axis: ABS_* value selecting the destination axis
+ * @src: Source input device to copy the abs settings from
+ * @src_axis: ABS_* value selecting the source axis
+ *
+ * Set absinfo for the selected destination axis by copying it from
+ * the specified source input device's source axis.
+ * This is useful to e.g. setup a pen/stylus input-device for combined
+ * touchscreen/pen hardware where the pen uses the same coordinates as
+ * the touchscreen.
+ */
+void input_copy_abs(struct input_dev *dst, unsigned int dst_axis,
+		    const struct input_dev *src, unsigned int src_axis)
+{
+	/* src must have EV_ABS and src_axis set */
+	if (WARN_ON(!(test_bit(EV_ABS, src->evbit) &&
+		      test_bit(src_axis, src->absbit))))
+		return;
+
+	/*
+	 * input_alloc_absinfo() may have failed for the source. Our caller is
+	 * expected to catch this when registering the input devices, which may
+	 * happen after the input_copy_abs() call.
+	 */
+	if (!src->absinfo)
+		return;
+
+	input_set_capability(dst, EV_ABS, dst_axis);
+	if (!dst->absinfo)
+		return;
+
+	dst->absinfo[dst_axis] = src->absinfo[src_axis];
+}
+EXPORT_SYMBOL(input_copy_abs);
 
 /**
  * input_grab_device - grabs device for exclusive use

include/linux/input.h

···
 void input_alloc_absinfo(struct input_dev *dev);
 void input_set_abs_params(struct input_dev *dev, unsigned int axis,
 			  int min, int max, int fuzz, int flat);
+void input_copy_abs(struct input_dev *dst, unsigned int dst_axis,
+		    const struct input_dev *src, unsigned int src_axis);
 
 #define INPUT_GENERATE_ABS_ACCESSORS(_suffix, _item)			\
 static inline int input_abs_get_##_suffix(struct input_dev *dev,	\