On Tue, Nov 20, 2018 at 9:17 AM Marc Zyngier [off-list ref] wrote:

How does this change anything with the fact that the above code is
broken? 56 or 64 bit, you cannot read this counter with a single
access, or two. The canonical way of reading such a counter is
something like this:

        do {
                lo = readl_relaxed(LO);
                hi = readl_relaxed(HI);
        } while (hi != read_relaxed(HI));

To be fair, I have seen hardware that employ a logic latch
such that when a read access is done to the LO register,
the value of the whole counter is latched, also for the HI
register, so when you read the HI register in the second
step, it is never subject to wrapping. (Conversely reading
the HI before the LO will always give you insane values
:D)

However the above code should be fine unless you know
for sure the hardware was constructed with a clever latch.

Yours,
Linus Walleij