Connect a rotary encoder to STM32F4. Needs time base initialization on TIM4 but not on TIM8, no idea why

tim4.c

#include "stm32f4_discovery.h"
#include "arm_math.h"


/*
 - TIM4_CH1  pin (PB.06)
 - TIM4_CH2  pin (PB.07)
 */
void configureEncoder() {
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOB, ENABLE);
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM4, ENABLE);
    
    GPIO_InitTypeDef GPIO_InitStructure;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
    
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_6| GPIO_Pin_7;
    GPIO_Init(GPIOB, &GPIO_InitStructure);
    
    GPIO_PinAFConfig(GPIOB, GPIO_PinSource6, GPIO_AF_TIM4);
    GPIO_PinAFConfig(GPIOB, GPIO_PinSource7, GPIO_AF_TIM4);
    
    TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
    TIM_TimeBaseStructure.TIM_Period = 0xffff;
    TIM_TimeBaseStructure.TIM_Prescaler = 0;
    TIM_TimeBaseStructure.TIM_ClockDivision = 0;
    TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
    
    TIM_TimeBaseInit(TIM4, &TIM_TimeBaseStructure);
    
    /* Configure the timer */
    TIM_EncoderInterfaceConfig(TIM4, TIM_EncoderMode_TI12, TIM_ICPolarity_Rising, TIM_ICPolarity_Rising);
    
    /* TIM4 counter enable */
    TIM_Cmd(TIM4, ENABLE);
}

// TIM4 is used for encoder
int main(void)
{
    //enable FPU
    SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2));
    
    STM_EVAL_LEDInit(LED3);
    STM_EVAL_LEDInit(LED4);
    STM_EVAL_LEDInit(LED5);
    STM_EVAL_LEDInit(LED6);
    STM_EVAL_LEDOff(LED3);
    STM_EVAL_LEDOff(LED4);
    STM_EVAL_LEDOff(LED5);
    STM_EVAL_LEDOff(LED6);
    configureEncoder();
    STM_EVAL_LEDOn(LED3);
    while (1)
    {
    }
}

int32_t rT2() {
    return TIM_GetCounter(TIM4);
}

tim8.c

#include "stm32f4_discovery.h"
#include "arm_math.h"


/*
 - TIM8_CH1  pin (PC.06)
 - TIM8_CH2  pin (PC.07)
 */
void configureEncoder() {
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOC, ENABLE);
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_TIM8, ENABLE);
    
    GPIO_InitTypeDef GPIO_InitStructure;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
    
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_6| GPIO_Pin_7;
    GPIO_Init(GPIOC, &GPIO_InitStructure);
    
    GPIO_PinAFConfig(GPIOC, GPIO_PinSource6, GPIO_AF_TIM8);
    GPIO_PinAFConfig(GPIOC, GPIO_PinSource7, GPIO_AF_TIM8);
    
    /* Configure the timer */
    TIM_EncoderInterfaceConfig(TIM8, TIM_EncoderMode_TI12, TIM_ICPolarity_Rising, TIM_ICPolarity_Rising);
    
    /* TIM8 counter enable */
    TIM_Cmd(TIM8, ENABLE);
}

// TIM8 is used for encoder
int main(void)
{
    //enable FPU
    SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2));
    
    STM_EVAL_LEDInit(LED3);
    STM_EVAL_LEDInit(LED4);
    STM_EVAL_LEDInit(LED5);
    STM_EVAL_LEDInit(LED6);
    STM_EVAL_LEDOff(LED3);
    STM_EVAL_LEDOff(LED4);
    STM_EVAL_LEDOff(LED5);
    STM_EVAL_LEDOff(LED6);
    configureEncoder();
    STM_EVAL_LEDOn(LED3);
    while (1)
    {
    }
}

int32_t rT2() {
    return TIM_GetCounter(TIM8);
}

Thank you for your great efforts.
please i want to ask you about this function
rT2() what is it??