On 2019-09-27 13:44, Benjamin GAIGNARD wrote:
On 9/27/19 2:41 PM, Marc Zyngier wrote:
On 2019-09-27 13:36, Benjamin GAIGNARD wrote:
On 9/27/19 1:22 PM, Marc Zyngier wrote:
On 2019-09-27 09:48, Benjamin Gaignard wrote:
Adding always-on makes arm arch_timer claim to be an high
resolution
timer.
That is possible because power mode won't stop clocking the
timer.
The "always-on" is not about the clock. It is about the
comparator.
The clock itself is *guaranteed* to always tick. If it didn't,
that'd be
an integration bug, and a pretty bad one.
What you're claiming here is that your CPU never enters a
low-power
mode?
Ever? I find this very hard to believe.
Furthermore, claiming that always-on is the way to force the
arch-timer
to be an hrtimer is factually wrong. This is what happens *if*
this is
the only timer in the system. The only case this is true is for
virtual
machines. Anything else has a global timer somewhere that will
allow
the arch timers to be used as an hrtimer.
I'm pretty sure you too have a global timer somewhere in your
system.
Enable it, and enjoy hrtimers without having to lie about the
properties
of your system! ;-)
Hi Marc,
This SoC doesn't have any other global timer. Use arch_time is the
only
we have to provide hrtimer on this system.
And you don't have any form of power management either? What happens
when
your CPU goes into idle? If your system does any form of power
management
*and* doesn't have a separate timer, it is remarkably broken.
Even in low-power modes this timer is always powered and clocked so
it
is working fine.
You're missing the point again. It is not about the clock, but the
comparator
that is internal to the CPU, and not functional when the CPU is in its
lowest
power mode. See also the verbiage in [1] (44.3 STGEN functional
description),
which indicates that the clock source actually dies in low-power mode
(going
against the architecture which mandates it to be always-on).
Also, coming back to your earlier assertion ("This SoC doesn't have any
other
global timer"): The documentation at [1] shows at least 17 timers that
could
be used and avoid this dirty hack.
So for what it is worth, NAK to this patch.
M.
[1] https://www.st.com/resource/en/reference_manual/dm00366355.pdf
--
Jazz is not dead. It just smells funny...
