On 12/04/2012 12:05 AM, Joseph Lo wrote: > On Tue, 2012-12-04 at 02:31 +0800, Stephen Warren wrote: >> On 12/02/2012 08:00 PM, 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". >> >>> diff --git a/arch/arm/mach-tegra/sleep-tegra20.S b/arch/arm/mach-tegra/sleep-tegra20.S >> >>> +/* >>> + * 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) >> >> A link to a description of that algorithm would be useful. >> > This algorithm was very famous. It can be very easy to find on the > internet. But if you want the link of the paper, it may not be > available. It usually need to access the paper as a member. > > Can you accept a link from wikipedia? > http://en.wikipedia.org/wiki/Peterson's_algorithm#cite_ref-1 Yes, that's fine (although I'd drop the anchor from the URL; i.e. remove "#cite_ref-1"). >>> + * on cpu 0: >>> + * SCRATCH38 = r2 = flag[0] >>> + * SCRATCH39 = r3 = flag[1] >>> + * on cpu1: >>> + * SCRATCH39 = r2 = flag[1] >>> + * SCRATCH38 = r3 = flag[0] >> >> That implies that r2/r3 are used for different purposes on the 2 CPUs, >> and/or shadow the values of different registers. However, I see nothing >> in the code which is conditional on cpu ID. > > here: > + 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 > It's proper to say that the SCRATCH38/SCRATCH39 are used for dedicate > cpu lock flag on the 2 CPUs. Doh. I overlooked the "cpu_id" invocation. -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
