* Jon Hunter <jon-hunter@xxxxxx> [120911 09:26]: > > On 09/10/2012 07:58 PM, Tony Lindgren wrote: > > * Jon Hunter <jon-hunter@xxxxxx> [120910 15:00]: > >> > >> On 09/07/2012 05:22 PM, Tony Lindgren wrote: > >>> * Jon Hunter <jon-hunter@xxxxxx> [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 -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
