linux-kernel - Re: [PATCH 1/2] ARM: oxnas: Add OX820 SMP support

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Message-ID: <20161031081827.GQ1041@n2100.armlinux.org.uk>
Date:   Mon, 31 Oct 2016 08:18:27 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Neil Armstrong <narmstrong@...libre.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org,
        linux-oxnas@...ts.tuxfamily.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] ARM: oxnas: Add OX820 SMP support

On Mon, Oct 17, 2016 at 11:34:32AM +0200, Neil Armstrong wrote:
> On 10/17/2016 11:06 AM, Arnd Bergmann wrote:
> > On Monday, October 17, 2016 10:43:02 AM CEST Neil Armstrong wrote:
> > This seems to have been copied from plat-versatile, but is really
> > not needed here since you apparently have proper hardware support for
> > starting up the CPUs.
> Yes it seems.
> 
> > 
> > Any reason you can't just write to the cpu_ctrl register
> > once and keep going without that whole holding_pen loop
> > and spinlock?
> I suppose but I did not find any good examples except the plat-versatile code.
> I will try some simpler code.

There's plenty of examples - most ARM SMP platforms in the kernel now
do not blindly copy the versatile code.  You only have to go looking
for arch/arm/*/platsmp.c files to find them.

I'm not sure what you'd call a "good example" - maybe the imx code?
arch/arm/mach-imx/platsmp.c can't be simpler:

static int imx_boot_secondary(unsigned int cpu, struct task_struct *idle)
{
        imx_set_cpu_jump(cpu, v7_secondary_startup);
        imx_enable_cpu(cpu, true);
        return 0;
}

I guess the difficult thing is to understand what each of those called
functions does... though the function names give a very accurate clue
there.

and because plat-versatile is almost entirely software-based, it's
easy to understand and follow, _despite_ being completely broken
for things like PM and kexec (which, the platform does not support.)

-- 
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