Re: [PATCH] ARM: dts: stm32: Enable high resolution timer

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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...



[Index of Archives]     [Device Tree Compilter]     [Device Tree Spec]     [Linux Driver Backports]     [Video for Linux]     [Linux USB Devel]     [Linux PCI Devel]     [Linux Audio Users]     [Linux Kernel]     [Linux SCSI]     [XFree86]     [Yosemite Backpacking]


  Powered by Linux