how to avoid caching when using mmap()

I'm writing a driver in petalinux for a device in my FPGA and I have implemented the mmap function in order to control the device in the user space. My problem is that, also if I'm using

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

in the mmap function and MAP_SHARED flag in the user application, it seems that the cache is enabled.

The test I did is to write a value (say 5) to a specific register of my mmaped device that actually stores only the least significant bit of the data coming from the AXI bus. If I read immediately after the write operation, I expect to read 1 (this happened when using a bare metal application on Microblaze), instead I read 5. However, the value is correctly wrote in the register, because what has to happen....happens.

Thanks in advance.

Solution

Based on what was discussed in the question comments, the address pointer being assigned here:

address = mmap(NULL, PAGE_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

wasn't declared with the type qualifier volatile, allowing the compiler to preform assumptions over it, leading to potential compile time optimizations over the read/write operations.