[REPOST] [PATCH] ARM: MMU: add a Non-cacheable Normal executablememory type

[Paul Walmsley]

Hello,

Any comments on this patch?  It is an unusual use for executable, uncached 
memory; but for our purposes, it should be much faster than a full cache 
flush, and conceptually cleaner than strongly-ordered executable memory, 
which appear to be our only other options in this situation.

We've tested this on ARMv7 (OMAP3).  If anyone would care to doublecheck 
the setup for architectures below ARMv7, it would be much appreciated.

This patch has been entered into rmk's parch system as patch 5356/1. 
Unfortunately, we can't merge any of the OMAP3 CORE DVFS code until this 
patch, or one like it, is merged.  Any reason why this patch, or one like 
it, shouldn't be merged?

thanks for any review and comment,


- Paul

---------- Forwarded message ----------
Date: Mon, 15 Dec 2008 14:07:07 -0700 (MST)
From: Paul Walmsley <paul@xxxxxxxxx>
To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxxxxx
Cc: linux-omap@xxxxxxxxxxxxxxx, r-woodruff2@xxxxxx
Subject: [PATCH] ARM: MMU: add a Non-cacheable Normal executable memory type


This patch adds a Non-cacheable Normal ARM executable memory type, 
MT_MEMORY_NONCACHED.

On OMAP3, this is used for rapid dynamic voltage/frequency scaling in the 
VDD2 voltage domain.  OMAP3's SDRAM controller (SDRC) is in the VDD2 
voltage domain, and its clock frequency must change along with voltage.  
The SDRC clock change code cannot run from SDRAM itself, since SDRAM 
accesses are paused during the clock change.  So the current 
implementation of the DVFS code executes from OMAP on-chip SRAM, aka "OCM 
RAM."

If the OCM RAM pages are marked as Cacheable, the ARM cache controller 
will attempt to flush dirty cache lines to the SDRC, so it can fill those 
lines with OCM RAM instruction code.  The problem is that the SDRC is 
paused during DVFS, and so any SDRAM access causes the ARM MPU subsystem 
to hang.

TI's original solution to this problem was to mark the OCM RAM sections as 
Strongly Ordered memory, thus preventing caching.  This is overkill: since 
the memory is marked as non-bufferable, OCM RAM writes become quite 
needlessly slow.  The idea of "Strongly Ordered SRAM" is also conceptually 
disturbing.  Previous LAKML list discussion is here:

   http://www.spinics.net/lists/arm-kernel/msg54312.html

Now this memory type MT_MEMORY_NONCACHED is used by a future patch for OCM 
RAM.

Signed-off-by: Paul Walmsley <paul@xxxxxxxxx>
Cc: Richard Woodruff <r-woodruff2@xxxxxx>
---
 arch/arm/include/asm/mach/map.h |    1 +
 arch/arm/mm/mmu.c               |   23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+), 0 deletions(-)

diff --git a/arch/arm/include/asm/mach/map.h b/arch/arm/include/asm/mach/map.h
index 39d949b..58cf91f 100644
--- a/arch/arm/include/asm/mach/map.h
+++ b/arch/arm/include/asm/mach/map.h
@@ -26,6 +26,7 @@ struct map_desc {
 #define MT_HIGH_VECTORS		8
 #define MT_MEMORY		9
 #define MT_ROM			10
+#define MT_MEMORY_NONCACHED	11
 
 #ifdef CONFIG_MMU
 extern void iotable_init(struct map_desc *, int);
diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 7f36c82..9ad6413 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -242,6 +242,10 @@ static struct mem_type mem_types[] = {
 		.prot_sect = PMD_TYPE_SECT,
 		.domain    = DOMAIN_KERNEL,
 	},
+	[MT_MEMORY_NONCACHED] = {
+		.prot_sect = PMD_TYPE_SECT | PMD_SECT_AP_WRITE,
+		.domain    = DOMAIN_KERNEL,
+	},
 };
 
 const struct mem_type *get_mem_type(unsigned int type)
@@ -405,9 +409,28 @@ static void __init build_mem_type_table(void)
 		kern_pgprot |= L_PTE_SHARED;
 		vecs_pgprot |= L_PTE_SHARED;
 		mem_types[MT_MEMORY].prot_sect |= PMD_SECT_S;
+		mem_types[MT_MEMORY_NONCACHED].prot_sect |= PMD_SECT_S;
 #endif
 	}
 
+	/*
+	 * Non-cacheable Normal - intended for memory areas that must
+	 * not cause dirty cache line writebacks when used
+	 */
+	if (cpu_arch >= CPU_ARCH_ARMv6) {
+		if (cpu_arch >= CPU_ARCH_ARMv7 && (cr & CR_TRE)) {
+			/* Non-cacheable Normal is XCB = 001 */
+			mem_types[MT_MEMORY_NONCACHED].prot_sect |=
+				PMD_SECT_BUFFERED;
+		} else {
+			/* For both ARMv6 and non-TEX-remapping ARMv7 */
+			mem_types[MT_MEMORY_NONCACHED].prot_sect |=
+				PMD_SECT_TEX(1);
+		}
+	} else {
+		mem_types[MT_MEMORY_NONCACHED].prot_sect |= PMD_SECT_BUFFERABLE;
+	}
+
 	for (i = 0; i < 16; i++) {
 		unsigned long v = pgprot_val(protection_map[i]);
 		protection_map[i] = __pgprot(v | user_pgprot);

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