linux-kernel - Re: [PATCH v6 1/5] powerpc/85xx: implement hardware timebase sync

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Date:	Tue, 26 Jun 2012 16:45:55 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Kumar Gala <galak@...nel.crashing.org>
CC:	Zhao Chenhui <chenhui.zhao@...escale.com>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
	<leoli@...escale.com>
Subject: Re: [PATCH v6 1/5] powerpc/85xx: implement hardware timebase sync

On 06/26/2012 09:03 AM, Kumar Gala wrote:
> 
> On Jun 26, 2012, at 5:25 AM, Zhao Chenhui wrote:
> 
>> Do hardware timebase sync. Firstly, stop all timebases, and transfer
>> the timebase value of the boot core to the other core. Finally,
>> start all timebases.
>>
>> Only apply to dual-core chips, such as MPC8572, P2020, etc.
>>
>> Signed-off-by: Zhao Chenhui <chenhui.zhao@...escale.com>
>> Signed-off-by: Li Yang <leoli@...escale.com>
>> ---
>> Changes for v6:
>> * added 85xx_TB_SYNC
>> * added isync() after set_tb()
>> * removed extra entries from mpc85xx_smp_guts_ids
> 
> Why only on dual-core chips?  Is this because of something related to
> 2 cores, or related to corenet vs non-corenet SoCs and how turning
> on/off the timebase works in the SOC?

Some parts are due to corenet versus non-corenet, such as the actual
register you write to to disable/enable the timebase.

There's also a two-core assumption in the synchronization code which
I've complained about multiple times -- although on closer inspection it
looks like this is done under cpu_add_remove_lock, and we can assume
that there's only one core at a time in take_timebase(), regardless of
how many cores are in the system.

-Scott

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