Re: [PATCH] hw/timer/armv7m_systick: Update clock source before enabling timer

[Richard Petri]

On Tue, Feb 01 2022, Peter Maydell <peter.maydell@linaro.org> wrote:
> Thanks, you've saved me a debugging session! I had a bug report about
> a problem with the systick timer a couple of days back, but I hadn't yet
> got round to investigating it, and now I don't have to, because this
> patch fixes the reported failure :-)

Oh, I didn't see the issue in the tracker. I guess someone else played
around with the new machines at the same time :-) A quick workaround on
the guest code side btw: set the clock source and then add the enable
flag with a separate write.

And if you still want to do a debug session: I think there is another
related problem. The `systick_reset` function sets the right clock
source, but I guess at the time of calling reset the `cpuclk` doesn't
have the right value (probably zero)? I was confused at first, because
the reset code suggests that everything is OK from the start, but that
doesn't seem to be the case. I don't have a good enough overview of the
qemu sources to know what is called when. But even if the reset would be
right, first setting the clock source and then en/disabling the timer is
the better order IMO.

/Richard