Re: [PATCH 08/10] ARM: OMAP: Clean-up timer posted mode support

[Tony Lindgren <tony@atomide.com>]

* Jon Hunter <jon-hunter@ti.com> [120911 09:26]:
> 
> On 09/10/2012 07:58 PM, Tony Lindgren wrote:
> > * Jon Hunter <jon-hunter@ti.com> [120910 15:00]:
> >>
> >> On 09/07/2012 05:22 PM, Tony Lindgren wrote:
> >>> * Jon Hunter <jon-hunter@ti.com> [120905 12:05]:
> >>>> The dmtimer functions to read and write the dmtimer registers are currently
> >>>> defined as follows ...
> >>>>
> >>>> static inline u32 __omap_dm_timer_read(struct omap_dm_timer *timer, u32 reg,
> >>>> 						int posted);
> >>>> static inline void __omap_dm_timer_write(struct omap_dm_timer *timer,
> >>>> 						u32 reg, u32 val, int posted);
> >>>>
> >>>> The posted variable indicates if the timer is configured to use the posted mode
> >>>> when performing register accesses. The posted mode configuration of the dmtimer
> >>>> is stored in the omap_dm_timer structure that is also being passed to the above
> >>>> functions and therefore we do not need to pass the posted variable separately.
> >>>> Therefore, simplify the above functions by removing the posted variable as an
> >>>> argument as this is not necessary.
> >>>
> >>> I believe the reason for passing the posted flag was to optimize out some
> >>> functions from the timer code as that's being run all the time.
> >>>
> >>> Care to check the assembly before and after this patch for the timer
> >>> functions with objdump -d to make sure it does not add tons of bloat
> >>> there?
> >>
> >> Hi Tony,
> >>
> >> Thanks for the details here. I see that makes sense and that the
> >> compiler could take advantage of this as the functions are inlined.
> >>
> >> I have taken a look at the disassembled output using objdump as you
> >> mentioned. What I see is ...
> >>
> >> 1. For dmtimer.c the impact appears negligible, the total number of
> >>    lines outputted by objdump only changed by 8 with (1215 lines) and
> >>    without (1207 lines) the patch applied.
> >>
> >> 2. For timer.c the impact is greater. I see that
> >>    omap2_gp_timer_set_next_event() increased by 6 instructions from 29
> >>    to 35. clocksource_read_cycles() increased by 2 instructions 15 to
> >>    17 instructions. dmtimer_read_sched_clock() increased by 2
> >>    instructions from 17 to 19. omap2_gp_timer_set_mode() increased by
> >>    21 instructions from 102 to 123.
> >>
> >> I imagine that we are mainly concerned about
> >> omap2_gp_timer_set_next_event(), clocksource_read_cycles() and
> >> dmtimer_read_sched_clock() as these will be called often. Therefore, I
> >> am not sure if you wish to drop this patch.
> > 
> > Well does it at lots of new ldr to the critical code?
> 
> For the omap2_gp_timer_set_next_event() function it adds 3 load
> instructions, increasing the number of possible loads from 11 to 14.
> 
> For the clocksource_read_cycles() function it adds 1 load instruction,
> increasing the number of possible loads from 7 to 8.
> 
> For the dmtimer_read_sched_clock() function, I don't see any additional
> loads, but instructions added are a tst and beq instruction.
> 
> >> By the way, if we do drop this patch, I would then need to fix the
> >> setting of the posted variable in mach-omap2/timer.c for clock-source in
> >> the case where a dmtimer is used. Today the code assumes that for
> >> clock-source and clock-events posted mode is always used. However, with
> >> the errata i103/i767 we will disable posted mode for clock-source on
> >> omap2/3/4/5/am33xx devices.
> > 
> > Yes I see, I guess that means just adding a new systimer entry?
> 
> Actually, I think we can avoid that by not using posted mode for
> clock-source timers at all. Posted mode only benefits the clock-events
> timers that are configured often.
> 
> The benefit of not using posted mode for clock-source timers and setting
> the "posted" parameter to 0, will really allow the compiler to optimise
> the clocksource_read_cycles() and dmtimer_read_sched_clock() quite a
> bit. So this could be a nice optimisation.

OK up to you, but maybe run some benchmarks first to figure out what
makes most sense? Updating the timer used to be a bottleneck earlier.

Regards,

Tony