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/testf... https://github.com/SietseAchterop/Batradio/blob/master/batradio_module/batra... 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