linux-kernel - Re: [PATCH RESEND 0/4] SMP support for Broadcom NSP

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Date:	Fri, 6 Nov 2015 17:55:02 +0530
From:	Kapil Hali <kapilh@...adcom.com>
To:	Hauke Mehrtens <hauke@...ke-m.de>,
	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Rob Herring <robh+dt@...nel.org>, Pawel Moll <pawel.moll@....com>,
	"Mark Rutland" <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, Ray Jui <rjui@...adcom.com>,
	Scott Branden <sbranden@...adcom.com>,
	Jon Mason <jonmason@...adcom.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	"Gregory Fong" <gregory.0xf0@...il.com>,
	Lee Jones <lee@...nel.org>, Heiko Stuebner <heiko@...ech.de>,
	Kever Yang <kever.yang@...k-chips.com>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Olof Johansson <olof@...om.net>,
	"Paul Walmsley" <paul@...an.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Chen-Yu Tsai <wens@...e.org>, <devicetree@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>,
	<bcm-kernel-feedback-list@...adcom.com>
Subject: Re: [PATCH RESEND 0/4] SMP support for Broadcom NSP



On 11/6/2015 1:55 AM, Hauke Mehrtens wrote:
> On 11/05/2015 10:34 AM, Russell King - ARM Linux wrote:
>> On Thu, Nov 05, 2015 at 12:51:17AM -0500, Kapil Hali wrote:
>>> Hi,
>>>
>>> This series adds SMP support for Broadcom's Northstar Plus SoC.
>>>
>>> There are similar SMP enablement methods for many ARMv7 bsed SoCs.
>>> BCM NSP SoC, has a typical such mechanism - after power-on, the
>>> secondary core is held in a standby state, primary core provides a
>>> startup address for the secondary core and wakes it up. Booting of
>>> the secondary core is serialized using pen_release global variable.
>>
>> Why do you need the pen_release stuff?  The above implies that you
>> have only one secondary core, and you can control when it comes out
>> of standby state.
>>
>> Please, don't assume that the pen_release stuff is any kind of recommended
>> or standardised system.  It isn't.  It's a hack for ARMs evaluation
>> platforms.
>>
> 
> I tried to remove the pen code because I also thought/hoped that it is
> useless, but the 2. CPU did not boot any more after I removed it. I do
> not know the internals of SoC, but it looks like this is needed.
> 
> I described it here:
> http://www.spinics.net/lists/arm-kernel/msg452178.html
> 
> I removed this comparison and the jump afterwards ("cmp     r7, r0") and
> the 2. CPU did not boot any more. Is this pen stuff some kind of
> workaround for some bug in the silicon?
> 
pen stuff is not a work around for any bug in BCM NSP silicon. It was the
mechanism to bring-up SMP on some of the ARM based SoCs and many silicon
vendors seem to consider it as a generic mechanism for controlled bring-up
of SMP. But as is clear from Russel King's comments, it is not a 
standardized recommended method.
Also, I removed the pen_release method and it works on BCM NSP SoCs. I 
will add the changes in the next patch set.
> Hauke
> 
Thanks,
Kapil Hali
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