Dropping "depends on SMP" for HAVE_ARM_TWD -- take 2

Mason slash.tmp at free.fr
Sat Oct 3 02:49:41 PDT 2015

On 03/10/2015 11:32, Marc Zyngier wrote:
> On Sat, 3 Oct 2015 00:34:02 +0100
> Måns Rullgård wrote:
>> Mason writes:
>>> My port requires the TWD to function. So I'm using
>>> select HAVE_ARM_TWD (no "if SMP") to have it work
>>> even on MULTIPLATFORM UP configs.
>> You could just let it use another timer in the non-smp case if you
>> didn't have that weird aversion towards using my code.
> I have no idea what your code does, but using the timer that is
> integrated into the core (just like on any other A9 implementation)
> feels like the right thing to do, unless the HW is too broken to use
> TWD.
> Another timer is a nice thing to have (it probably comes in handy for
> suspend/resume, and it may have a much better resolution), but this
> seems like icing on the cake at this point. Let's see the cake first.


I have a quick question for you :-)

As I stated several months ago, my goal is to produce the simplest
port possible, which is why I chose the TWD instead of platform
timers. At one point, I was also considering the global timer.
(I have a Cortex A9 MPCore.) But as far as I could tell, the
code for the global timer does not handle CPU frequency changes
(while smp_twd.c does), is that correct?

AFAIU, TWD timers and global timer "tick" at the frequency of
PERIPHCLK (which is CPUCLK/2 in my implementation). Therefore,
code needs to be specifically written to handle cpufreq events.
Is that a correct understanding?

As for you comment about resolution, nominal PERIPHCLK is 600 MHz.
Hard to beat that kind of resolution, I think.

Have a nice week end.


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