[PATCH v4 4/4] powerpc/85xx: add sysfs for pw20 state and altivec idle

[Bhushan Bharat-R65777]

> -----Original Message-----
> From: Wang Dongsheng-B40534
> Sent: Thursday, September 26, 2013 8:02 AM
> To: Wood Scott-B07421
> Cc: Bhushan Bharat-R65777; linuxppc-dev at lists.ozlabs.org
> Subject: RE: [PATCH v4 4/4] powerpc/85xx: add sysfs for pw20 state and altivec
> idle
> 
> 
> 
> > -----Original Message-----
> > From: Wood Scott-B07421
> > Sent: Thursday, September 26, 2013 1:57 AM
> > To: Wang Dongsheng-B40534
> > Cc: Bhushan Bharat-R65777; Wood Scott-B07421; linuxppc-
> > dev at lists.ozlabs.org
> > Subject: Re: [PATCH v4 4/4] powerpc/85xx: add sysfs for pw20 state and
> > altivec idle
> >
> > On Wed, 2013-09-25 at 03:10 -0500, Wang Dongsheng-B40534 wrote:
> > >
> > > > -----Original Message-----
> > > > From: Bhushan Bharat-R65777
> > > > Sent: Wednesday, September 25, 2013 2:23 PM
> > > > To: Wang Dongsheng-B40534; Wood Scott-B07421
> > > > Cc: linuxppc-dev at lists.ozlabs.org; Wang Dongsheng-B40534
> > > > Subject: RE: [PATCH v4 4/4] powerpc/85xx: add sysfs for pw20 state
> > > > and altivec idle
> > > >
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Linuxppc-dev [mailto:linuxppc-dev-
> > > > > bounces+bharat.bhushan=freescale.com at lists.ozlabs.org] On Behalf
> > > > > bounces+Of Dongsheng
> > > > > Wang
> > > > > Sent: Tuesday, September 24, 2013 2:59 PM
> > > > > To: Wood Scott-B07421
> > > > > Cc: linuxppc-dev at lists.ozlabs.org; Wang Dongsheng-B40534
> > > > > Subject: [PATCH v4 4/4] powerpc/85xx: add sysfs for pw20 state
> > > > > and altivec idle
> > > > >
> > > > > From: Wang Dongsheng <dongsheng.wang at freescale.com>
> > > > >
> > > > > Add a sys interface to enable/diable pw20 state or altivec idle,
> > > > > and control the wait entry time.
> > > > >
> > > > > Enable/Disable interface:
> > > > > 0, disable. 1, enable.
> > > > > /sys/devices/system/cpu/cpuX/pw20_state
> > > > > /sys/devices/system/cpu/cpuX/altivec_idle
> > > > >
> > > > > Set wait time interface:(Nanosecond)
> > > > > /sys/devices/system/cpu/cpuX/pw20_wait_time
> > > > > /sys/devices/system/cpu/cpuX/altivec_idle_wait_time
> > > > > Example: Base on TBfreq is 41MHZ.
> > > > > 1~47(ns): TB[63]
> > > > > 48~95(ns): TB[62]
> > > > > 96~191(ns): TB[61]
> > > > > 192~383(ns): TB[62]
> > > > > 384~767(ns): TB[60]
> > > > > ...
> > > > >
> > > > > Signed-off-by: Wang Dongsheng <dongsheng.wang at freescale.com>
> > > > > ---
> > > > > *v4:
> > > > > Move code from 85xx/common.c to kernel/sysfs.c.
> > > > >
> > > > > Remove has_pw20_altivec_idle function.
> > > > >
> > > > > Change wait "entry_bit" to wait time.
> > > > >
> > > > >  arch/powerpc/kernel/sysfs.c | 291
> > > > > ++++++++++++++++++++++++++++++++++++++++++++
> > > > >  1 file changed, 291 insertions(+)
> > > > >
> > > > > diff --git a/arch/powerpc/kernel/sysfs.c
> > > > > b/arch/powerpc/kernel/sysfs.c index 27a90b9..23fece6 100644
> > > > > --- a/arch/powerpc/kernel/sysfs.c
> > > > > +++ b/arch/powerpc/kernel/sysfs.c
> > > > > @@ -85,6 +85,279 @@ __setup("smt-snooze-delay=",
> > > > > setup_smt_snooze_delay);
> > > > >
> > > > >  #endif /* CONFIG_PPC64 */
> > > > >
> > > > > +#ifdef CONFIG_FSL_SOC
> > > > > +#define MAX_BIT		63
> > > > > +
> > > > > +static u64 pw20_wt;
> > > > > +static u64 altivec_idle_wt;
> > > > > +
> > > > > +static unsigned int get_idle_ticks_bit(u64 ns) {
> > > > > +	u64 cycle;
> > > > > +
> > > > > +	cycle = div_u64(ns, 1000 / tb_ticks_per_usec);
> > > >
> > > > When tb_ticks_per_usec  > 1000 (timebase frequency > 1GHz) then
> > > > this will always be ns, which is not correct, no?
> >
> > Actually it'll be a divide by zero in that case.
> >
> tb_ticks_per_usec = ppc_tb_freq / 1000000; Means TB freq should be more than
> 1MHZ.
> 
> if ppc_tb_freq less than 1000000, the tb_ticks_per_usec will be a divide by
> zero.
> If this condition is established, I think kernel cannot work as a normal.
> 
> So I think we need to believe that the variable is not zero.

We do believe it is non-zero but greater than 1000 :)

> And I think TB freq
> should not less than 1MHZ on PPC platform, because if TB freq less than 1MHZ,
> the precision time will become very poor and system response time will be
> slower.

Not sure what you are describing here related to divide by zero we are mentioning.
You are talking about if tb_ticks_per_usec is ZERO and we are talking about if (1000/tb_ticks_per_usec) will be zero.

BTW, div_u64() handle the case where divider is zero.

> 
> > > "1000 / tb_ticks_per_usec" means nsec_ticks_per_tb
> > >
> > > If timebase frequency > 1GHz, this should be "tb_ticks_per_usec / 1000"
> > and to get tb_ticks_per_nsec.
> > > This should be changed to "cycle = ns * tb_ticks_per_nsec;"
> > >
> > > But at present we do not have such a platform that timebase
> > > frequency more than 1GHz. And I think it is not need to support such a
> situation.
> > > Because we have no environment to test it.
> >
> > You can test it by hacking a wrong timebase frequency in and seeing
> > what the calculation does.
> >
> > Or do something like this:
> >
> > 	if (ns >= 10000)
	^^^

> > 		cycle = ((ns + 500) / 1000) * tb_ticks_per_usec;
> > 	else
> > 		cycle = div_u64((u64)ns * tb_ticks_per_usec, 1000);
> >
> We cannot do this, because if (ns+500) < 1000, we cannot get the entry bit,
> it'll always zero bit.

There is a if condition of ns >= 10000, so ns+500 can not be less than 1000.

> 
> We must to use per_nsec_tb_ticks, like my code 1000 / tb_ticks_per_usec.
> 
> > ...which can be tested just by varying ns.
> >
> > > If later there will be more than 1GHZ platform at that time to add
> > > this
> > support.
> >
> Yes, I agree this point. :)

One should agree with himself :)

-Bharat

> 
> -dongsheng
> 
> > There almost certainly won't be timebases that run that fast, but
> > divide by zero is a rather nasty way of responding if such a thing does
> happen.
> >
> > -Scott
> >