[PATCH] ARM: zynq: wfi exit on same cpu is valid

[Russell King - ARM Linux]

On Tue, Jun 04, 2013 at 03:10:17PM +0100, Russell King - ARM Linux wrote:
> On Tue, Jun 04, 2013 at 01:58:31PM +0200, Daniel Lezcano wrote:
> > On 06/04/2013 01:39 PM, Amit Kucheria wrote:
> > > I'm curious why it is called pen_release. :) Is there some historical
> > > link to some HW lines?
> > 
> > I tried to figure out the same but I did not found any information on
> > that. I assumed the name could be referring to a simplified mutual
> > exclusion algorithm from the 'Dining philosophers problem' [1] where the
> > fork is a pen.
> 
> Where it comes from is the original ARM SMP patches from early 2000, which
> everyone has blindly copied with no thought about what they're doing.  This
> is why I'm totally against any consolidation of this code, because I'm of
> the opinion that _no one_ other than the ARM Ltd development platforms
> should be using it.
> 
> "pen" means "holding pen".  It comes about because early on in the SMP
> development, ARM SMP platforms had four CPUs, and it was only possible to
> release all three secondary CPUs from the boot loader simultaneously to
> a common piece of code.
> 
> As the kernel was not able to serialize the release of each CPU, ARM Ltd
> worked around this problem by having all the CPUs jump to assembly code
> which "holds" the CPUs which we didn't want to boot yet, and the CPUs
> are released one at a time by setting pen_release to the hardware CPU
> number.
> 
> Modern platforms either have just one secondary CPU, or they have a way
> to control the reset/power to the secondary CPU.  This makes the holding
> pen entirely redundant, and such platforms should _not_ make use of any
> kind of holding pen.

And yes, indeed, zynq can control the secondary CPU:

void zynq_slcr_cpu_start(int cpu)
{
        /* enable CPUn */
        writel(SLCR_A9_CPU_CLKSTOP << cpu,
               zynq_slcr_base + SLCR_A9_CPU_RST_CTRL);
        /* enable CLK for CPUn */
        writel(0x0 << cpu, zynq_slcr_base + SLCR_A9_CPU_RST_CTRL);
}

void zynq_slcr_cpu_stop(int cpu)
{
        /* stop CLK and reset CPUn */
        writel((SLCR_A9_CPU_CLKSTOP | SLCR_A9_CPU_RST) << cpu,
               zynq_slcr_base + SLCR_A9_CPU_RST_CTRL);
}

So there's no need for the pen.  There's no need for the low power crap
in hotplug.c, there's no need for the pen in hotplug.c.  You just arrange
for the secondary CPU to have its clock stopped and reset when it is
taken offline.

Hotplugging a CPU back in _should_ be no different from its initial
bringup into the kernel.