Changeset 8316547f in mainline for kernel/arch/arm32/src/cpu/cpu.c

Timestamp:

2012-09-19T11:21:26Z (12 years ago)

Author:

Jan Vesely <jano.vesely@…>

Branches:

lfn, master, serial, ticket/834-toolchain-update, topic/msim-upgrade, topic/simplify-dev-export

Children:

3896974

Parents:

a57b751

Message:

arm32: Add comments on control bits

File:

• kernel/arch/arm32/src/cpu/cpu.c (modified) (2 diffs)

kernel/arch/arm32/src/cpu/cpu.c

@@ -97 +97 @@
 }
 
-/** Does nothing on ARM. */
+/** Enables unaligned access and caching for armv6+ */
 void cpu_arch_init(void)
 {
@@ -107 +107 @@
         );
         
-        /* Turn off tex remap */
+        /* Turn off tex remap, RAZ ignores writes prior to armv7 */
         control_reg &= ~CP15_R1_TEX_REMAP_EN;
-        /* Turn off accessed flag */
+        /* Turn off accessed flag, RAZ ignores writes prior to armv7 */
         control_reg &= ~(CP15_R1_ACCESS_FLAG_EN | CP15_R1_HW_ACCESS_FLAG_EN);
-        /* Enable unaligned access (U bit is armv6 only) */
+        /* Enable unaligned access, RAZ ignores writes prior to armv6
+         * switchable on armv6, RAO ignores writes on armv7,
+         * see ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition
+         * L.3.1 (p. 2456) */
         control_reg |= CP15_R1_UNALIGNED_EN;
-        /* Disable alignment checks */
+        /* Disable alignment checks, this turns unaligned access to undefined,
+         * unless U bit is set. */
         control_reg &= ~CP15_R1_ALIGN_CHECK_EN;
-        /* Enable caching */
+        /* Enable caching, On arm prior to armv7 there is only one level
+         * of caches. Data cache is coherent.
+         * "This means that the behavior of accesses from the same observer to
+         * different VAs, that are translated to the same PA
+         * with the same memory attributes, is fully coherent."
+         *    ARM Architecture Reference Manual ARMv7-A and ARMv7-R Edition
+         *    B3.11.1 (p. 1383)
+         * ICache coherency is elaborate on in barrier.h.
+         * We are safe to turn these on.
+         */
         control_reg |= CP15_R1_CACHE_EN | CP15_R1_INST_CACHE_EN;