From ad0e669b922c7790182cf19f8015b30e23ad9499 Mon Sep 17 00:00:00 2001 From: Nikolai Kondrashov Date: Sun, 13 Oct 2013 15:09:52 +0300 Subject: HID: Fix unit exponent parsing again Revert some changes done in 774638386826621c984ab6994439f474709cac5e. Revert all changes done in hidinput_calc_abs_res as it mistakingly used "Unit" item exponent nibbles to affect resolution value. This wasn't breaking resolution calculation of relevant axes of any existing devices, though, as they have only one dimension to their units and thus 1 in the corresponding nible. Revert to reading "Unit Exponent" item value as a signed integer in hid_parser_global to fix reading specification-complying values. This fixes resolution calculation of devices complying to the HID standard, including Huion, KYE, Waltop and UC-Logic graphics tablets which have their report descriptors fixed by the drivers. Explanations follow. There are two "unit exponents" in HID specification and it is important not to mix them. One is the global "Unit Exponent" item and another is nibble values in the global "Unit" item. See 6.2.2.7 Global Items. The "Unit Exponent" value is just a signed integer and is used to scale the integer resolution unit values, so fractions can be expressed. The nibbles of "Unit" value are used to select the unit system (nibble 0), and presence of a particular basic unit type in the unit formula and its *exponent* (or power, nibbles 1-6). And yes, the latter is in two complement and zero means absence of the unit type. Taking the representation example of (integer) joules from the specification: [mass(grams)][length(centimeters)^2][time(seconds)^-2] * 10^-7 the "Unit Exponent" would be -7 (or 0xF9, if stored as a byte) and the "Unit" value would be 0xE121, signifying: Nibble Part Value Meaning ----- ---- ----- ------- 0 System 1 SI Linear 1 Length 2 Centimeters^2 2 Mass 1 Grams 3 Time -2 Seconds^-2 To give the resolution in e.g. hundredth of joules the "Unit Exponent" item value should have been -9. See also the examples of "Unit" values for some common units in the same chapter. However, there is a common misunderstanding about the "Unit Exponent" value encoding, where it is assumed to be stored the same as nibbles in "Unit" item. This is most likely due to the specification being a bit vague and overloading the term "unit exponent". This also was and still is proliferated by the official "HID Descriptor Tool", which makes this mistake and stores "Unit Exponent" as such. This format is also mentioned in books such as "USB Complete" and in Microsoft's hardware design guides. As a result many devices currently on the market use this encoding and so the driver should support them. Signed-off-by: Nikolai Kondrashov Acked-by: Benjamin Tissoires Signed-off-by: Jiri Kosina --- drivers/hid/hid-core.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) (limited to 'drivers/hid/hid-core.c') diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c index 5a8c01112a2..a9ca93c1192 100644 --- a/drivers/hid/hid-core.c +++ b/drivers/hid/hid-core.c @@ -319,7 +319,7 @@ static s32 item_sdata(struct hid_item *item) static int hid_parser_global(struct hid_parser *parser, struct hid_item *item) { - __u32 raw_value; + __s32 raw_value; switch (item->tag) { case HID_GLOBAL_ITEM_TAG_PUSH: @@ -370,10 +370,11 @@ static int hid_parser_global(struct hid_parser *parser, struct hid_item *item) return 0; case HID_GLOBAL_ITEM_TAG_UNIT_EXPONENT: - /* Units exponent negative numbers are given through a - * two's complement. - * See "6.2.2.7 Global Items" for more information. */ - raw_value = item_udata(item); + /* Many devices provide unit exponent as a two's complement + * nibble due to the common misunderstanding of HID + * specification 1.11, 6.2.2.7 Global Items. Attempt to handle + * both this and the standard encoding. */ + raw_value = item_sdata(item); if (!(raw_value & 0xfffffff0)) parser->global.unit_exponent = hid_snto32(raw_value, 4); else -- cgit v1.2.3-70-g09d2