Re: [PATCH] save and restore etm state across core OFF modes

[Nishanth Menon <menon.nishanth@gmail.com>]

Alexander Shishkin said the following on 01/13/2010 05:36 AM:
> On Tue, Jan 12, 2010 at 04:08:23 -0600, Nishanth Menon wrote:
>   
>> Alexander Shishkin had written, on 01/12/2010 03:46 PM, the following:
>>     
>>> On Tue, Jan 12, 2010 at 01:04:04 -0800, Tony Lindgren wrote:
>>>       
>>>> * Nishanth Menon <nm@ti.com> [100112 09:31]:
>>>>         
>>>>> Alexander Shishkin had written, on 01/12/2010 11:30 AM, the following:
>>>>>           
>>>>>> On Tue, Jan 12, 2010 at 11:13:13 -0600, Nishanth Menon wrote:
>>>>>>             
>>>>>>> Alexander Shishkin had written, on 01/12/2010 11:04 AM, the following:
>>>>>>>               
>>>>>>>> diff --git a/arch/arm/mach-omap2/sleep34xx.S b/arch/arm/mach-omap2/sleep34xx.S
>>>>>>>> index 69521be..0a5ec86 100644
>>>>>>>> --- a/arch/arm/mach-omap2/sleep34xx.S
>>>>>>>> +++ b/arch/arm/mach-omap2/sleep34xx.S
>>>>>>>>                 
>>>>> [...]
>>>>>           
>>>>>>>> 	/* Store current cpsr*/
>>>>>>>> 	mrs	r2, cpsr
>>>>>>>> 	stmia	r8!, {r2}
>>>>>>>> @@ -520,6 +616,7 @@ clean_caches:
>>>>>>>> 	cmp	r9, #1 /* Check whether L2 inval is required or not*/
>>>>>>>> 	bne	skip_l2_inval
>>>>>>>> clean_l2:
>>>>>>>> +#if 0
>>>>>>>>                 
>>>>>>> my aversion to #if 0 kicks in here :(.. do we have an alternative
>>>>>>> like using the CONFIG_ENABLE_OFF_MODE_JTAG_ETM_DEBUG or something
>>>>>>> else?
>>>>>>>               
>>>>>> Fair enough. I could replace it with "#if !defined(...)" as the first
>>>>>> thing that comes to mind. This way it will only take disabling the
>>>>>> config option to catch any possible regressions in between. Does this
>>>>>> sound reasonable?
>>>>>>             
>>>>> sounds ok to me.. unless folks have ideas coz of clean_l2 label..
>>>>> more comments might be useful before a rev2 of the patch..
>>>>>           
>>>> The best solution would be to be able to toggle this via sysfs or
>>>> debugfs by swapping the sram code for idle loop when JTAG support
>>>> is needed.
>>>>         
>>> Well, if you say, compile the ETM driver in, this will be needed most of
>>> the time.
>>>
>>>       
>> I can think of reasons for an against a sysfs entry (as part of
>> discussion -warning lot of self contradictions below- but I think
>> might save a bit of back and froth ;)):
>>
>> for sysfs entry:
>> a) save and restore will have additional latency when you save a
>> chunk such as EMU domain regs - this will not be needed in
>> production phones, disabling it might pop up surprises
>> 	counter: having a disabled defconfig allows relevant folks to
>> 	enable on a need basis
>> 		counter to counter: what do you do when a user reports
>> 		an issue in a release and you'd want to debug it with 		
>> 		ETM on his platform other than doing a rebuild?
>>     
>
> Well, my intention is to have it enabled for most of the cases only having
> it disabled for testing purposes.
>   
with a sysfs you can go either way, with proper #ifdeferry, you can get 
the best of all worlds I guess.. I know in one of the products, a 
similar patch was not taken in due to introduction of additional 
scratchpad space and latencies - so there are folks who would like this 
and those who would like to see this not present in the binary they 
flash to thier device.

>   
>> b) mostly a debug support -> only for blokes using ETM/JTAG
>> interfaces - not everyone can afford these (no offense to openOCD
>> guys - but they are still a bit away from being able to debug kernel
>> yet on OMAP)..
>>     
>
> Not really, you can still make use of ETM without any additional hardware
> attached.
For the unitiated (like me) link from Alex's offline email[1].

Regards,
Nishanth Menon

[1]

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=c5d6c7708c3e58015b2e4e13e6cea02c8567a94e