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

[Scott Wood <scottwood@freescale.com>]

On Thu, 2013-09-26 at 01:18 -0500, Wang Dongsheng-B40534 wrote:
> 
> > -----Original Message-----
> > From: Bhushan Bharat-R65777
> > Sent: Thursday, September 26, 2013 12:23 PM
> > To: Wang Dongsheng-B40534; Wood Scott-B07421
> > Cc: linuxppc-dev@lists.ozlabs.org
> > Subject: RE: [PATCH v4 4/4] powerpc/85xx: add sysfs for pw20 state and
> > altivec idle
> > 
> > 
> > 
> > > -----Original Message-----
> > > From: Wang Dongsheng-B40534
> > > Sent: Thursday, September 26, 2013 8:02 AM
> > > To: Wood Scott-B07421
> > > Cc: Bhushan Bharat-R65777; linuxppc-dev@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@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@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@lists.ozlabs.org] On
> > > > > > > bounces+Behalf Of Dongsheng
> > > > > > > Wang
> > > > > > > Sent: Tuesday, September 24, 2013 2:59 PM
> > > > > > > To: Wood Scott-B07421
> > > > > > > Cc: linuxppc-dev@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@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@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.

How?  When I checked yesterday it looked like div_u64() mapped to a
hardware division on 64-bit targets.  In any case it's not good to rely
on such behavior.

> cycle = div_u64(ns, 1000 / tb_ticks_per_usec);
> For this, I think we were discussing the two issues:
> 
> 1. Scott talking about, when the tb_ticks_per_usec is a zero.

No I wasn't.  I was talking about when tb_ticks_per_usec > 1000, and
thus "1000 / tb_ticks_per_usec" is zero.  You said that the result would
be ns in that case.

> 2. You are talking about 1000/tb_ticks_per_usec is a zero.
> This situation is about TB freq > 1GHZ.
> 
> I will fix this issue. Like I said before,
> "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;""
> 
> #define TB_FREQ_1GHZ	1000
> 
> If (tb_ticks_per_usec > TB_FREQ_1GHZ)
> 	cycle = ns * (tb_ticks_per_usec / 1000);
> else
> 	cycle = div_u64(ns, 1000 / tb_ticks_per_usec);
> 
> how about this? :)

I suggested an alternative to satisfy your complaint that it's hard to
test one of those if/else branches.

Plus, your version will be quite inaccurate if (e.g.) tb_ticks_per_usec
is 501, or 1999.

-Scott