[ARM] 5422/1: 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 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

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

Cc: Richard Woodruff <[e-mail]>
Signed-off-by: Paul Walmsley <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

arch/arm/include/asm/mach/map.h

···
 #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);

arch/arm/mm/mmu.c

···
 		.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)
···
 		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++) {