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.
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.
Regards
Santosh
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