On 11/07/2012 04:10 PM, Santosh Shilimkar wrote: > On Wednesday 07 November 2012 01:01 PM, Jon Hunter wrote: >> When using a DMTIMER as the clock-source timer, posted mode >> configuration of >> the DMTIMER is used. Posted mode is only benefical when configuring >> timers as >> it allows writes to be posted and does not stall the CPU until the >> write is >> complete. The clock-source timer is only configured once on boot and >> so using >> posted mode has no benefit. In fact, by using posted mode, it adds >> overhead >> to reading the timer. Therefore, by default disable posted mode for >> DMTIMERs >> used for clock-source. >> >> Using objdump this change reduces the function clocksource_read_cycles() >> function from a total of 15 instructions (including 3 branches and 7 >> loads) >> to 5 instructions (including 1 branch and 3 loads). Please note that >> before >> the minimum number of instructions that would be executed when calling >> clocksource_read_cycles() would be 9 instructions (including 2 >> branches and 5 >> loads) where as now it will always be 5 instructions. >> >> This change also reduces the function dmtimer_read_sched_clock() >> function from >> a total of 17 instructions (including 4 branches and 8 loads) to 6 >> instructions >> (including 1 branch and 4 loads). Please note that before the minimum >> number of >> instructions that would be executed when calling >> dmtimer_read_sched_clock() >> would be 11 instructions (including 2 branches and 6 loads) where as >> now it >> will always be 6 instructions. >> > This isn't right way to calculate the penalty of posted mode. Non-posted > mode can results in 100 of cycles wait over interconnect > and that can not be justified with few instructions savings. Right, I see your point. Non-posted reads are going to add re-synchronisation overhead. However, our hands are tied here because of errata i103 we can't use posted mode for reading the counter. So may be I should squash this change with patch #3 and just make this part of the errata workaround. I can always re-visit later and do some profiling to see what the optimal configuration should be. >> This change removes the configuration of posted mode from the >> __omap_dm_timer_reset() function and adds a new function called >> omap_dm_timer_enable_posted() for enabling posted mode. Hence, call >> omap_dm_timer_enable_posted() where ever we are calling >> __omap_dm_timer_reset(). >> >> Signed-off-by: Jon Hunter <jon-hunter@xxxxxx> >> --- > But clock-source doesn't involve much into writes so patch as > such is fine. Yes writes are only on init. Cheers Jon -- 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
