STM8 as SPI slave can't send back data

I have build a prototype board with a STM8L, and I want it to be used and configured as a SPI slave. I am testing it with a raspberry pi as master.

I use the lib provided by ST called "STM8 Standard Peripherals Library" for this, but the documentation is very poor and doesn't expain how to do this...

I can send data from the Raspberry Pi with no issue and receive it on the STM8 but I can't send back any data to the raspberry from the STM8 on MISO.

Is anybody known how I can send back some data to the Raspberry Pi master? Where is my mistake?

Here is the main code:

void main(void)
{
  // GPIO
  GPIO_Init(GPIOA, GPIO_Pin_7, GPIO_Mode_Out_PP_Low_Fast);
  CLK_Config();
  

  // Set the MOSI and SCK at high level 
  GPIO_ExternalPullUpConfig(GPIOB, GPIO_Pin_6 | GPIO_Pin_5, ENABLE);
  
  SPI_DeInit(SPI1);

  SPI_Init(SPI1, SPI_FirstBit_LSB, SPI_BaudRatePrescaler_2, SPI_Mode_Slave,
           SPI_CPOL_Low, SPI_CPHA_2Edge, SPI_Direction_2Lines_FullDuplex,
           SPI_NSS_Hard, (uint8_t)0x07);

  SPI_BiDirectionalLineConfig(SPI1, SPI_Direction_Tx);


  // Enable SPI
  SPI_Cmd(SPI1, ENABLE);


  /* Infinite loop */
  while (1)
  {
    while(SPI_GetFlagStatus(SPI1, SPI_FLAG_BSY));

    // SPI polling
    if(SPI_GetFlagStatus(SPI1, SPI_FLAG_RXNE) == SET) {

      while(SPI_GetFlagStatus(SPI1, SPI_FLAG_BSY));
      
      GPIO_ToggleBits(GPIOA, GPIO_Pin_7);
      
      uint8_t data = SPI_ReceiveData(SPI1);

      while(SPI_GetFlagStatus(SPI1, SPI_FLAG_BSY));

      // I can't send back data here, it doesn't work
      SPI_SendData(SPI1, 0xFA);

      uint8_t test = SPI1->DR;

      GPIO_ResetBits(GPIOA, GPIO_Pin_7);
    }
  }
}


static void CLK_Config(void)
{
  /* Select HSE as system clock source */
  CLK_SYSCLKSourceSwitchCmd(ENABLE);
  CLK_SYSCLKSourceConfig(CLK_SYSCLKSource_HSI);
  
  /*High speed external clock prescaler: 1*/
  CLK_SYSCLKDivConfig(CLK_SYSCLKDiv_2);

  while (CLK_GetSYSCLKSource() != CLK_SYSCLKSource_HSI)
  {}

  /* Enable SPI clock */
  CLK_PeripheralClockConfig(CLK_Peripheral_SPI1, ENABLE);
}

And the RPi simple code:

#include <iostream>
#include <wiringPi.h>
#include <wiringPiSPI.h>

using namespace std;

int main()
{
    wiringPiSetup();
    wiringPiSPISetup(0, 50000);

    unsigned char data[] = {0x5A};

    wiringPiSPIDataRW(0, data, 2);

    std::cout<<data<<std::endl;


    return 0;

Thank you for your help! :)

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