Subject: [PATCH 03/16] ARM: b.L: introduce helpers for platform coherency exit/setup

From: [email-masked] (Dave Martin)
Date: Fri, 11 Jan 2013 18:07:57 +0000

On Fri, Jan 11, 2013 at 11:16:18PM +0530, Santosh Shilimkar wrote:

[...]

> >+Originally created and documented by Dave Martin for Linaro Limited, in
> >+collaboration with Nicolas Pitre and Achin Gupta.
> >+
> Great write-up Dave!! I might have to do couple of more passes on it to
> get overall idea, but surely this documentation is good start for
> anybody reading/reviewing the big.LITTLE switcher code.

Thanks for reading through it.  Partly, this was insurance against me
forgetting how the code worked in between writing and posting it...
but this is all quite subtle code, so it felt important to document
it thoroughly.

> 
> >+Copyright (C) 2012  Linaro Limited
> >+Distributed under the terms of Version 2 of the GNU General Public
> >+License, as defined in linux/COPYING.
> >diff --git a/arch/arm/common/bL_entry.c b/arch/arm/common/bL_entry.c
> >index 41de0622de..1ea4ec9df0 100644
> >--- a/arch/arm/common/bL_entry.c
> >+++ b/arch/arm/common/bL_entry.c
> >@@ -116,3 +116,163 @@ int bL_cpu_powered_up(void)
> >  		platform_ops->powered_up();
> >  	return 0;
> >  }
> >+
> >+struct bL_sync_struct bL_sync;
> >+
> >+static void __sync_range(volatile void *p, size_t size)
> >+{
> >+	char *_p = (char *)p;
> >+
> >+	__cpuc_flush_dcache_area(_p, size);
> >+	outer_flush_range(__pa(_p), __pa(_p + size));
> >+	outer_sync();
> >+}
> >+
> >+#define sync_mem(ptr) __sync_range(ptr, sizeof *(ptr))
> >+
> >+/*
> /** as per kerneldoc.

Does kerneldoc not require the comment to be specially formatted?

I haven't played with that, so far.

> 
> >+ * __bL_cpu_going_down: Indicates that the cpu is being torn down.
> >+ *    This must be called at the point of committing to teardown of a CPU.
> >+ *    The CPU cache (SCTRL.C bit) is expected to still be active.
> >+ */
> >+void __bL_cpu_going_down(unsigned int cpu, unsigned int cluster)
> >+{
> >+	bL_sync.clusters[cluster].cpus[cpu].cpu = CPU_GOING_DOWN;
> >+	sync_mem(&bL_sync.clusters[cluster].cpus[cpu].cpu);
> >+}
> >+
> 
> [..]
> 
> >diff --git a/arch/arm/common/bL_head.S b/arch/arm/common/bL_head.S
> >index 9d351f2b4c..f7a64ac127 100644
> >--- a/arch/arm/common/bL_head.S
> >+++ b/arch/arm/common/bL_head.S
> >@@ -7,11 +7,19 @@
> >   * This program is free software; you can redistribute it and/or modify
> >   * it under the terms of the GNU General Public License version 2 as
> >   * published by the Free Software Foundation.
> >+ *
> >+ *
> >+ * Refer to Documentation/arm/big.LITTLE/cluster-pm-race-avoidance.txt
> >+ * for details of the synchronisation algorithms used here.
> >   */
> >
> >  #include <linux/linkage.h>
> >  #include <asm/bL_entry.h>
> >
> >+.if BL_SYNC_CLUSTER_CPUS
> >+.error "cpus must be the first member of struct bL_cluster_sync_struct"
> >+.endif
> >+
> >  	.macro	pr_dbg	cpu, string
> >  #if defined(CONFIG_DEBUG_LL) && defined(DEBUG)
> >  	b	1901f
> >@@ -52,12 +60,82 @@ ENTRY(bL_entry_point)
> >  2:	pr_dbg	r4, "kernel bL_entry_point\n"
> >
> >  	/*
> >-	 * MMU is off so we need to get to bL_entry_vectors in a
> >+	 * MMU is off so we need to get to various variables in a
> >  	 * position independent way.
> >  	 */
> >  	adr	r5, 3f
> >-	ldr	r6, [r5]
> >+	ldmia	r5, {r6, r7, r8}
> >  	add	r6, r5, r6			@ r6 = bL_entry_vectors
> >+	ldr	r7, [r5, r7]			@ r7 = bL_power_up_setup_phys
> >+	add	r8, r5, r8			@ r8 = bL_sync
> >+
> >+	mov	r0, #BL_SYNC_CLUSTER_SIZE
> >+	mla	r8, r0, r10, r8			@ r8 = bL_sync cluster base
> >+
> >+	@ Signal that this CPU is coming UP:
> >+	mov	r0, #CPU_COMING_UP
> >+	mov	r5, #BL_SYNC_CPU_SIZE
> >+	mla	r5, r9, r5, r8			@ r5 = bL_sync cpu address
> >+	strb	r0, [r5]
> >+
> >+	dsb
> Do you really need above dsb(). With MMU off, the the store should

The short answer is "maybe not".  Some of the barriers can be
eliminated; some can be demoted to DSBs.  Others may be required but
unnecessarily duplicated e.g., between bL_head.S and vlock.S.

> any way make it to the main memory, No ?

Yes, but this raises issues about precisely what the architecture
guarantees about memory ordering in these scenarios.  The only obvious
thing about that is that it's non-obvious.

Strongly-Ordered memory is not quite the same as having explicit
barriers everywhere.

I need to have a careful think, but it should be possible to optimise
a bit here.

> 
> >+
> >+	@ At this point, the cluster cannot unexpectedly enter the GOING_DOWN
> >+	@ state, because there is at least one active CPU (this CPU).
> >+
> >+	@ Check if the cluster has been set up yet:
> >+	ldrb	r0, [r8, #BL_SYNC_CLUSTER_CLUSTER]
> >+	cmp	r0, #CLUSTER_UP
> >+	beq	cluster_already_up
> >+
> >+	@ Signal that the cluster is being brought up:
> >+	mov	r0, #INBOUND_COMING_UP
> >+	strb	r0, [r8, #BL_SYNC_CLUSTER_INBOUND]
> >+
> >+	dsb
> Same comment.

Same answer... for now

Cheers
---Dave