Add absolute axis resolution calculation to the core HID layer, according to HID
specification v1.11 6.2.2.7 Global Items. Only exponent 1 length units for
X/Y/Z/RX/RY/RZ axis are supported for now.
Signed-off-by: Nikolai Kondrashov <spbnick@xxxxxxxxx>
---
The code tries to preserve the precision at the cost of possible overflow.
That's why radians are chosen for rotation resolution. The code is tested using
synthetic report descriptors and it remains to be seen how it will behave in the
wild.
Though devices correctly specifying their physical extents are pretty rare, this
code could also be used to decode resolution from fixed report descriptors of
such HID drivers as waltop and uclogic.
I plan to use it for retrieving tablet resolution in the xf86-input-wacom
driver.
drivers/hid/hid-input.c | 80 +++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 80 insertions(+), 0 deletions(-)
diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
index 0c2908f..deae566 100644
--- a/drivers/hid/hid-input.c
+++ b/drivers/hid/hid-input.c
@@ -149,6 +149,83 @@ static int hidinput_setkeycode(struct input_dev *dev,
}
+/**
+ * hidinput_calc_abs_res - calculate an absolute axis resolution
+ * @field: the HID report field to calculate resolution for
+ * @code: axis code
+ *
+ * The formula is:
+ * (logical_maximum - logical_minimum)
+ * resolution = ----------------------------------------------------------
+ * (physical_maximum - physical_minimum) * 10 ^ unit_exponent
+ *
+ * as seen in the HID specification v1.11 6.2.2.7 Global Items.
+ *
+ * Only exponent 1 length units are processed. Centimeters are converted to
+ * inches. Degrees are converted to radians.
+ */
+static __s32 hidinput_calc_abs_res(const struct hid_field *field, __u16 code)
+{
+ __s32 unit_exponent = field->unit_exponent;
+ __s32 logical_extents = field->logical_maximum -
+ field->logical_minimum;
+ __s32 physical_extents = field->physical_maximum -
+ field->physical_minimum;
+ __s32 prev;
+
+ /* Check if the extents are sane */
+ if (logical_extents <= 0 || physical_extents <= 0)
+ return 0;
+
+ /*
+ * Verify and convert units.
+ * See HID specification v1.11 6.2.2.7 Global Items for unit decoding
+ */
+ if (code == ABS_X || code == ABS_Y || code == ABS_Z) {
+ if (field->unit == 0x11) { /* If centimeters */
+ /* Convert to inches */
+ prev = logical_extents;
+ logical_extents *= 254;
+ if (logical_extents < prev)
+ return 0;
+ unit_exponent += 2;
+ } else if (field->unit != 0x13) { /* If not inches */
+ return 0;
+ }
+ } else if (code == ABS_RX || code == ABS_RY || code == ABS_RZ) {
+ if (field->unit == 0x14) { /* If degrees */
+ /* Convert to radians */
+ prev = logical_extents;
+ logical_extents *= 573;
+ if (logical_extents < prev)
+ return 0;
+ unit_exponent += 1;
+ } else if (field->unit != 0x12) { /* If not radians */
+ return 0;
+ }
+ } else {
+ return 0;
+ }
+
+ /* Apply negative unit exponent */
+ for (; unit_exponent < 0; unit_exponent++) {
+ prev = logical_extents;
+ logical_extents *= 10;
+ if (logical_extents < prev)
+ return 0;
+ }
+ /* Apply positive unit exponent */
+ for (; unit_exponent > 0; unit_exponent--) {
+ prev = physical_extents;
+ physical_extents *= 10;
+ if (physical_extents < prev)
+ return 0;
+ }
+
+ /* Calculate resolution */
+ return logical_extents / physical_extents;
+}
+
static void hidinput_configure_usage(struct hid_input *hidinput, struct hid_field *field,
struct hid_usage *usage)
{
@@ -541,6 +618,9 @@ mapped:
input_set_abs_params(input, usage->code, a, b, (b - a) >> 8, (b - a) >> 4);
else input_set_abs_params(input, usage->code, a, b, 0, 0);
+ input_abs_set_res(input, usage->code,
+ hidinput_calc_abs_res(field, usage->code));
+
/* use a larger default input buffer for MT devices */
if (usage->code == ABS_MT_POSITION_X && input->hint_events_per_packet == 0)
input_set_events_per_packet(input, 60);
--
1.7.1
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