struct - Python: writing to memory in a single operation

Question

I'm writing a userspace driver for accessing FPGA registers in Python 3.5 that mmaps the FPGA's PCI address space, obtains a memoryview to provide direct access to the memory-mapped register space, and then uses struct.pack_into("<I", ...) to write a 32-bit value into the selected 32-bit aligned address.

def write_u32(address, data):
    assert address % 4 == 0, "Address must be 32-bit aligned"
    path = path.lib.Path("/dev/uio0")
    file_size = path.stat().st_size
    with path.open(mode='w+b') as f:
        mv = memoryview(mmap.mmap(f.fileno(), file_size))
        struct.pack_into("<I", mv, address, data)

Unfortunately, it appears that struct.pack_into does a memset(buf, 0, ...) that clears the register before the actual value is written. By examining write operations within the FPGA, I can see that the register is set to 0x00000000 before the true value is set, so there are at least two writes across the PCI bus (in fact for 32-bit access there are three, two zero writes, then the actual data. 64-bit involves six writes). This causes side-effects with some registers that count the number of write operations, or some that "clear on write" or trigger some event when written.

I'd like to use an alternative method to write the register data in a single write to the memory-mapped register space. I've looked into ctypes.memmove and it looks promising (not yet working), but I'm wondering if there are other ways to do this.

Note that a register read using struct.unpack_from works perfectly.

Note that I've also eliminated the FPGA from this by using a QEMU driver that logs all accesses - I see the same double zero-write access before data is written.

Answer 1

Perhaps this would work as needed?

mv[address:address+4] = struct.pack("<I", data)

Update:

As seen from the comments, the code above does not solve the problem. The following variation of it does, however:

mv_as_int = mv.cast('I')
mv_as_int[address/4] = data

Unfortunately, precise understanding of what happens under the hood and why exactly memoryview behaves this way is beyond the capabilities of modern technology and will thus stay open for the researchers of the future to tackle.