From: jon-hunter@ti•com (Jon Hunter)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH V2 02/14] ARM: OMAP2+: Disable posted mode for the clocksource timer
Date: Wed, 7 Nov 2012 16:41:47 -0600 [thread overview]
Message-ID: <509AE3AB.8080008@ti.com> (raw)
In-Reply-To: <509ADC5F.7020307@ti.com>
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@ti•com>
>> ---
> But clock-source doesn't involve much into writes so patch as
> such is fine.
Yes writes are only on init.
Cheers
Jon
next prev parent reply other threads:[~2012-11-07 22:41 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-11-07 19:01 [PATCH V2 00/14] ARM: OMAP: DMTIMER fixes Jon Hunter
2012-11-07 19:01 ` [PATCH V2 01/14] ARM: OMAP: Add DMTIMER definitions for posted mode Jon Hunter
2012-11-07 22:04 ` Santosh Shilimkar
2012-11-07 22:11 ` Jon Hunter
2012-11-07 22:18 ` Santosh Shilimkar
2012-11-07 22:47 ` Jon Hunter
2012-11-08 16:52 ` Jon Hunter
2012-11-07 19:01 ` [PATCH V2 02/14] ARM: OMAP2+: Disable posted mode for the clocksource timer Jon Hunter
2012-11-07 22:10 ` Santosh Shilimkar
2012-11-07 22:41 ` Jon Hunter [this message]
2012-11-07 19:01 ` [PATCH V2 03/14] ARM: OMAP3+: Implement timer workaround for errata i103 and i767 Jon Hunter
2012-11-07 22:14 ` Santosh Shilimkar
2012-11-07 23:28 ` Jon Hunter
2012-11-07 23:43 ` Santosh Shilimkar
2012-11-08 16:55 ` Jon Hunter
2012-11-09 19:41 ` Jon Hunter
2012-11-07 19:01 ` [PATCH V2 04/14] ARM: OMAP: Fix timer posted mode support Jon Hunter
2012-11-07 22:21 ` Santosh Shilimkar
2012-11-07 19:01 ` [PATCH V2 05/14] ARM: OMAP3: Correct HWMOD DMTIMER SYSC register declarations Jon Hunter
2012-11-07 19:01 ` [PATCH V2 06/14] ARM: OMAP2/3: Define HWMOD software reset status for DMTIMERs Jon Hunter
2012-11-07 19:01 ` [PATCH V2 07/14] ARM: OMAP2+: Don't use __omap_dm_timer_reset() Jon Hunter
2012-11-07 19:01 ` [PATCH V2 08/14] ARM: OMAP: Fix dmtimer reset for timer1 Jon Hunter
2012-11-07 19:01 ` [PATCH V2 09/14] ARM: OMAP: Don't restore of DMTIMER TISTAT register Jon Hunter
2012-11-07 19:01 ` [PATCH V2 10/14] ARM: OMAP: Don't restore DMTIMER interrupt status register Jon Hunter
2012-11-07 19:01 ` [PATCH V2 11/14] ARM: OMAP: Fix spurious interrupts when using timer match feature Jon Hunter
2012-11-07 19:01 ` [PATCH V2 12/14] ARM: OMAP: Add dmtimer interrupt disable function Jon Hunter
2012-11-07 19:01 ` [PATCH V2 13/14] ARM: OMAP: Remove unnecessary call to clk_get() Jon Hunter
2012-11-07 19:01 ` [PATCH V2 14/14] ARM: OMAP: Remove __omap_dm_timer_set_source function Jon Hunter
2012-11-07 21:22 ` [PATCH V2 00/14] ARM: OMAP: DMTIMER fixes Tony Lindgren
2012-11-07 21:44 ` Jon Hunter
2012-11-07 21:52 ` Tony Lindgren
2012-11-07 22:27 ` Santosh Shilimkar
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