From: Chris Metcalf <cmetcalf@tilera.com>
Date: Tue, 9 Apr 2013 16:33:07 +0000 (-0400)
Subject: tile: comment assumption about __insn_mtspr for <asm/irqflags.h>
X-Git-Tag: v3.10-rc1~134^2~1
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=ffae3d0e3606b2e274c9ec1b969342d630b2ecae;p=karo-tx-linux.git

tile: comment assumption about __insn_mtspr for <asm/irqflags.h>

The arch_local_irq_save(), etc., routines are required to function
as compiler barriers.  They do, but it's subtle and requires knowing
that the gcc builtin __insn_mtspr() is marked as a memory clobber.
Provide a comment explaining the assumption.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
---

diff --git a/arch/tile/include/asm/irqflags.h b/arch/tile/include/asm/irqflags.h
index 241c0bb60b12..c96f9bbb760d 100644
--- a/arch/tile/include/asm/irqflags.h
+++ b/arch/tile/include/asm/irqflags.h
@@ -40,7 +40,15 @@
 #include <asm/percpu.h>
 #include <arch/spr_def.h>
 
-/* Set and clear kernel interrupt masks. */
+/*
+ * Set and clear kernel interrupt masks.
+ *
+ * NOTE: __insn_mtspr() is a compiler builtin marked as a memory
+ * clobber.  We rely on it being equivalent to a compiler barrier in
+ * this code since arch_local_irq_save() and friends must act as
+ * compiler barriers.  This compiler semantic is baked into enough
+ * places that the compiler will maintain it going forward.
+ */
 #if CHIP_HAS_SPLIT_INTR_MASK()
 #if INT_PERF_COUNT < 32 || INT_AUX_PERF_COUNT < 32 || INT_MEM_ERROR >= 32
 # error Fix assumptions about which word various interrupts are in