suse man page for struct_ff_envelope

STRUCT 
FF_ENVELOPE(9)						  Input Subsystem					     STRUCT FF_ENVELOPE(9)

NAME
       struct_ff_envelope - generic force-feedback effect envelope

SYNOPSIS
       struct ff_envelope {
	 __u16 attack_length;
	 __u16 attack_level;
	 __u16 fade_length;
	 __u16 fade_level;
       };

MEMBERS
       attack_length
	   duration of the attack (ms)

       attack_level
	   level at the beginning of the attack

       fade_length
	   duration of fade (ms)

       fade_level
	   level at the end of fade

DESCRIPTION
       The attack_level and fade_level are absolute values; when applying envelope force-feedback core will convert to positive/negative value
       based on polarity of the default level of the effect. Valid range for the attack and fade levels is 0x0000 - 0x7fff

COPYRIGHT
Kernel Hackers Manual 2.6.					     July 2010						     STRUCT FF_ENVELOPE(9)