[PATCH V2 2/6] ARM: tegra20: cpuidle: add powered-down state for secondary CPU

[Nicolas Pitre]

On Fri, 11 Jan 2013, Joseph Lo wrote:

> Hi Lorenzo,
> 
> On Wed, 2012-12-05 at 18:50 +0800, Lorenzo Pieralisi wrote:
> > On Wed, Dec 05, 2012 at 10:01:49AM +0000, Joseph Lo wrote:
> > > The powered-down state of Tegra20 requires power gating both CPU cores.
> > > When the secondary CPU requests to enter powered-down state, it saves
> > > its own contexts and then enters WFI. The Tegra20 had a limition to
> > > power down both CPU cores. The secondary CPU must waits for CPU0 in
> > > powered-down state too. If the secondary CPU be woken up before CPU0
> > > entering powered-down state, then it needs to restore its CPU states
> > > and waits for next chance.
> > >
> > > Be aware of that, you may see the legacy power state "LP2" in the code
> > > which is exactly the same meaning of "CPU power down".
> > >
> > > Based on the work by:
> > > Colin Cross <ccross at android.com>
> > > Gary King <gking at nvidia.com>
> > >
> > > Signed-off-by: Joseph Lo <josephl at nvidia.com>
> > > +
> > > +#ifdef CONFIG_PM_SLEEP
> > > +/*
> > > + * tegra_pen_lock
> > > + *
> > > + * spinlock implementation with no atomic test-and-set and no coherence
> > > + * using Peterson's algorithm on strongly-ordered registers
> > > + * used to synchronize a cpu waking up from wfi with entering lp2 on idle
> > > + *
> > > + * SCRATCH37 = r1 = !turn (inverted from Peterson's algorithm)
> > > + * on cpu 0:
> > > + * SCRATCH38 = r2 = flag[0]
> > > + * SCRATCH39 = r3 = flag[1]
> > > + * on cpu1:
> > > + * SCRATCH39 = r2 = flag[1]
> > > + * SCRATCH38 = r3 = flag[0]
> > > + *
> > > + * must be called with MMU on
> > > + * corrupts r0-r3, r12
> > > + */
> > > +ENTRY(tegra_pen_lock)
> > > +       mov32   r3, TEGRA_PMC_VIRT
> > > +       cpu_id  r0
> > > +       add     r1, r3, #PMC_SCRATCH37
> > > +       cmp     r0, #0
> > > +       addeq   r2, r3, #PMC_SCRATCH38
> > > +       addeq   r3, r3, #PMC_SCRATCH39
> > > +       addne   r2, r3, #PMC_SCRATCH39
> > > +       addne   r3, r3, #PMC_SCRATCH38
> > > +
> > > +       mov     r12, #1
> > > +       str     r12, [r2]               @ flag[cpu] = 1
> > > +       dsb
> > > +       str     r12, [r1]               @ !turn = cpu
> > > +1:     dsb
> > > +       ldr     r12, [r3]
> > > +       cmp     r12, #1                 @ flag[!cpu] == 1?
> > > +       ldreq   r12, [r1]
> > > +       cmpeq   r12, r0                 @ !turn == cpu?
> > > +       beq     1b                      @ while !turn == cpu && flag[!cpu] == 1
> > > +
> > > +       mov     pc, lr                  @ locked
> > > +ENDPROC(tegra_pen_lock)
> > > +
> > > +ENTRY(tegra_pen_unlock)
> > > +       dsb
> > > +       mov32   r3, TEGRA_PMC_VIRT
> > > +       cpu_id  r0
> > > +       cmp     r0, #0
> > > +       addeq   r2, r3, #PMC_SCRATCH38
> > > +       addne   r2, r3, #PMC_SCRATCH39
> > > +       mov     r12, #0
> > > +       str     r12, [r2]
> > > +       mov     pc, lr
> > > +ENDPROC(tegra_pen_unlock)
> > 
> > There is an ongoing work to make this locking scheme for MMU/coherency off
> > paths ARM generic, and we do not want to merge yet another platform specific
> > locking mechanism. I will point you to the patchset when it hits LAK.
> > 
> 
> You did mention there is an ARM generic locking scheme for MMU/coherency
> off case before. Do you mean the patch below?
> 
> https://patchwork.kernel.org/patch/1957911/
> https://patchwork.kernel.org/patch/1957901/
> 
> I gave it a review today. Looks it can't fit our usage for CPU idle
> powered-down mode on Tegra20.
> 
> The generic mechanism only can be used when CPUs in non coherent world.
> But our usage needs the mechanism could be used in both coherent and non
> coherent case. OK. The case is we need to sync the status about the CPU1
> was ready to power down. In this case, the CPU0 is still in coherent
> world but the CPU1 isn't. So we need the locking scheme could be still
> safe in this situation.

Those patches above are dealing with CPUs coming back up.  If you look 
at the rest of the series, especially the DCSCB one providing real usage 
example, you'll see that we do have a mix of coherent and non-coherent 
states that need to coordinate amongst themselves.

> And the proposed scheme looks only for b.L system?

It was initiated for b.L hence the name, but that is applicable to other 
systems as well.


Nicolas