From 33b96d934902f96e901b72ac18bbc47afad1ac20 Mon Sep 17 00:00:00 2001 From: Elias Vanderstuyft Date: Wed, 14 Oct 2015 17:29:37 -0700 Subject: Input: document and check on implicitly defined FF_MAX_EFFECTS There is an undocumented upper bound for the total number of ff effects: FF_GAIN (= 96). This can be found as follows: - user: write(EV_FF, effect_id, iterations) calls kernel: ff->playback(effect_id, ...): starts effect "effect_id" - user: write(EV_FF, FF_GAIN, gain) calls kernel: ff->set_gain(gain, ...): sets gain A collision occurs when effect_id equals FF_GAIN. According to input_ff_event(), FF_GAIN is the smallest value where a collision occurs. Therefore the greatest safe value for effect_id is FF_GAIN - 1, and thus the total number of effects should never exceed FF_GAIN. Define FF_MAX_EFFECTS as FF_GAIN and check on this limit in ff-core. Signed-off-by: Elias Vanderstuyft Signed-off-by: Dmitry Torokhov --- include/uapi/linux/input.h | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'include/uapi/linux/input.h') diff --git a/include/uapi/linux/input.h b/include/uapi/linux/input.h index a8e74b45dbe2..4f9f2a0f5573 100644 --- a/include/uapi/linux/input.h +++ b/include/uapi/linux/input.h @@ -414,6 +414,14 @@ struct ff_effect { #define FF_GAIN 0x60 #define FF_AUTOCENTER 0x61 +/* + * ff->playback(effect_id = FF_GAIN) is the first effect_id to + * cause a collision with another ff method, in this case ff->set_gain(). + * Therefore the greatest safe value for effect_id is FF_GAIN - 1, + * and thus the total number of effects should never exceed FF_GAIN. + */ +#define FF_MAX_EFFECTS FF_GAIN + #define FF_MAX 0x7f #define FF_CNT (FF_MAX+1) -- cgit v1.2.3-59-g8ed1b