MMAP buffer kernel writes are not seen by user space - linux-kernel

Question

i have a kernel driver which shares a buffer with the user space layer.
Everything seemed to work fine in my VM prototype (Ubuntu, Kernel 5.4) but when i moved my code to the target (same kernel but this is an embedded distro) I can clearly see that Kernel writes to the buffer (using memcpy, or memset) are not reflected in the User space side of the buffer.
Note that, i use direct buffer accesses on both sides. There is no concurrency issue, as the Kernel writes to, then the user space reads from.
I ended up believing this is a cache issue ... as the same code works perfectly in my VM.
The buffer size is 4 * PAGE_SIZE.
It is allocated as follows:
int _size = (SFP_BUFFER_SIZE + (PAGE_SIZE-1)) & ~(PAGE_SIZE-1);
input_buffer = (char*) kzalloc (_size, GFP_KERNEL); // aligned on page boundary
if (!input_buffer) {
dev_dbg(&dev, "open/ENOMEM (input_buffer)\n");
status = -ENOMEM;
goto err_all
When mmap'ing, i used the following code pattern:
vma->vm_ops = &fpgadrv_vm_ops;
vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
pfn = virt_to_phys((void*)(input_buffer)) >> PAGE_SHIFT;
if (remap_pfn_range (vma, vma->vm_start, pfn, size, vma->vm_page_prot))
{
printk(KERN_DEBUG "remap page range failed\n");
return -EAGAIN;
}
User space code, and kernel code user memcpy to update the buffer. Note also that I cannot use write/read entry points, as they are already used for very specific operations.
The user code is calling mmap as follows:
buf = mmap(NULL, BUF_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, device_fd, 0);
if (buf == MAP_FAILED)
{
perror("USERDRV:cannot mmap");
return -1; // for testing, ignore the return code and continue
}
and upon IOCTL call, the kernel would fill up the mmap buffer as follows:
case IOCTL_RESET:
printk(KERN_DEBUG "FPGADRV: IOCTL RESET");
// reset the buffer (zero + put back the signature)
memset(input_buffer, 0xA5, SFP_BUFFER_SIZE);
memcpy((void*)(input_buffer), (void*)signature, 10);
break;
Is there something more i should do to make sure the pages are not cached (assuming this is the cause of my pb) ?
Thanks,
Jacques