mmap not working in device driver

[s.achte...@rug.nl]

  Hello list,

I created a simple misc-device platform device driver, including a test 
program, see
    
https://github.com/SietseAchterop/Batradio/blob/master/batradio_module/testfiqtimer.c
    
https://github.com/SietseAchterop/Batradio/blob/master/batradio_module/batradio_client.c

This on an raspberry zero with the latest kernel from  
https://github.com/raspberrypi/linux,
compiled from source using arm-linux-gnueabihf-gcc (GCC) 8.3.0.

The driver only implements the mmap and ioctl functions.
An area in the driver (fiq_buf) is made available to the user program via mmap.
The problem is that changing data (status value) in the driver via ioctl, is 
NOT reflected in the user program.

Here a few snippets from the driver:

From the init function:

  batradio_data = devm_kzalloc(&pdev->dev,
                               sizeof(*batradio_data),
                               GFP_KERNEL);
  if (!batradio_data)
    return -ENOMEM;

  batradio_data->fiq_base = devm_kzalloc(&pdev->dev,
                                         FIQ_BUFFER_SIZE,   // 256*1024
                                         GFP_KERNEL);
  if (!batradio_data->fiq_base) {
    dev_err(&pdev->dev, "Couldn't allocate memory!\n");
    return -ENOMEM;
  }

  fiq_buf = (struct fiq_buffer *)batradio_data->fiq_base;
  fiq_buf->status = 55;

And from the mmap function:

  size_t size = vma->vm_end - vma->vm_start;
  unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;

  if (offset + size > FIQ_BUFFER_SIZE)
    return -EINVAL;

  offset += virt_to_phys(batradio_data->fiq_base);

  vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);

  if (remap_pfn_range(vma,
                      vma->vm_start,
                      offset >> PAGE_SHIFT,
                      size,
                      vma->vm_page_prot)) {
    return -EAGAIN;
  }

And finally from the client program:

  fiq_fd = open(FIQ_PATH, O_RDWR);
  if (!fiq_fd) {
    ret = errno;
    perror("Couldn't open batradio device, error %d", errno);
    exit(-1);
  }

  fiq_addr = mmap(NULL, FIQ_BUFFER_SIZE, PROT_READ | PROT_WRITE,
                  MAP_SHARED, fiq_fd, 0);
  if (fiq_addr == MAP_FAILED) {
    ret = errno;
    perror("Couldn't map the fiq buffer");
    exit(-2);
  }
  fiq_buf = (struct fiq_buffer*)fiq_addr;

  printf("status %d\n", fiq_buf->status);


All very standard I think.
Why are changes in fiq_buf->status in the driver not shown in the client 
program?

  Thanks in advance,
      Sietse

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