linux-kernel - Re: [PATCH v2 02/11] HID: core: fix unit exponent parsing

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Date:	Mon, 29 Oct 2012 20:05:54 +0100
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Benjamin Tissoires <benjamin.tissoires@...il.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Stephane Chatty <chatty@...c.fr>, linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/11] HID: core: fix unit exponent parsing

Hi Benjamin,

> HID spec details special values for the HID field unit exponent.
> Basically, the range [0x8..0xf] correspond to [-8..-1].
> 
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...il.com>
> ---

Standard two's complement, in other words?

>  drivers/hid/hid-core.c  | 10 +++++++++-
>  drivers/hid/hid-input.c | 19 +++++++++++++------
>  2 files changed, 22 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
> index 9904776..6bde6e4 100644
> --- a/drivers/hid/hid-core.c
> +++ b/drivers/hid/hid-core.c
> @@ -315,6 +315,7 @@ static s32 item_sdata(struct hid_item *item)
>  
>  static int hid_parser_global(struct hid_parser *parser, struct hid_item *item)
>  {
> +	__u32 raw_value;
>  	switch (item->tag) {
>  	case HID_GLOBAL_ITEM_TAG_PUSH:
>  
> @@ -365,7 +366,14 @@ static int hid_parser_global(struct hid_parser *parser, struct hid_item *item)
>  		return 0;
>  
>  	case HID_GLOBAL_ITEM_TAG_UNIT_EXPONENT:
> -		parser->global.unit_exponent = item_sdata(item);
> +		/* Units exponent negative numbers are given through a special
> +		 * table.
> +		 * See "6.2.2.7 Global Items" for more information. */
> +		raw_value = item_udata(item);
> +		if ((raw_value >> 3) == 1)
> +			parser->global.unit_exponent = raw_value - 16;
> +		else
> +			parser->global.unit_exponent = raw_value;

I beleive the function you want is snto32(value, 4).

>  		return 0;
>  
>  	case HID_GLOBAL_ITEM_TAG_UNIT:
> diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
> index 1b0adc3..fc9f2b5 100644
> --- a/drivers/hid/hid-input.c
> +++ b/drivers/hid/hid-input.c
> @@ -215,7 +215,7 @@ __s32 hidinput_calc_abs_res(const struct hid_field *field, __u16 code)
>  					field->logical_minimum;
>  	__s32 physical_extents = field->physical_maximum -
>  					field->physical_minimum;
> -	__s32 prev;
> +	__s32 prev, tmp_exponent;
>  
>  	/* Check if the extents are sane */
>  	if (logical_extents <= 0 || physical_extents <= 0)
> @@ -235,17 +235,24 @@ __s32 hidinput_calc_abs_res(const struct hid_field *field, __u16 code)
>  	case ABS_MT_TOOL_Y:
>  	case ABS_MT_TOUCH_MAJOR:
>  	case ABS_MT_TOUCH_MINOR:
> -		if (field->unit == 0x11) {		/* If centimeters */
> +		if (field->unit & 0xffffff00)		/* Not a length */
> +			return 0;
> +		tmp_exponent = field->unit >> 4;
> +		if ((tmp_exponent >> 3) == 1)
> +			tmp_exponent -= 16;

Ditto.

> +		switch (field->unit & 0xf) {
> +		case 0x1:				/* If centimeters */
>  			/* Convert to millimeters */
> -			unit_exponent += 1;
> -		} else if (field->unit == 0x13) {	/* If inches */
> +			unit_exponent += tmp_exponent;
> +			break;
> +		case 0x3:				/* If inches */
>  			/* Convert to millimeters */
>  			prev = physical_extents;
>  			physical_extents *= 254;
>  			if (physical_extents < prev)
>  				return 0;
> -			unit_exponent -= 1;
> -		} else {
> +			unit_exponent += tmp_exponent - 2;
> +		default:
>  			return 0;
>  		}
>  		break;
> -- 
> 1.7.11.7
> 

Thanks,
Henrik
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