commit 0120ddaf7f3cc04d97ca113a3ced3a7f5c6daae2
author Ian Tessier <itessier@google.com> Tue Jul 07 08:57:02 2020 -0700
committer CQ Bot Account <commit-bot@chromium.org> Thu Sep 03 17:48:25 2020 +0000
tree f87d5910a56dcfebcc6641cbc38782550a102ad5
parent a8a5b4013b42be41b4d7027113675fe9273a97f0

pw_boot_armv7m: Add barriers for enabling the FPU

The barriers (as recommended per the STM32 programming manual) ensure
that the FPU is correctly enabled before continuing and potentially
executing any FPU instructions, however rare that may be during startup.

Change-Id: I846c87c5fd72ac297ed6556e57cb7ca672d6d83a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/13360
Commit-Queue: Ian Tessier <itessier@google.com>
Commit-Queue: Keir Mierle <keir@google.com>
Reviewed-by: Keir Mierle <keir@google.com>

targets/stm32f429i-disc1/boot.cc

diff --git a/targets/stm32f429i-disc1/boot.cc b/targets/stm32f429i-disc1/boot.cc
index 439d0a9..a0d0fc2 100644
--- a/targets/stm32f429i-disc1/boot.cc
+++ b/targets/stm32f429i-disc1/boot.cc
@@ -35,6 +35,19 @@
   volatile uint32_t& arm_v7m_cpacr =
       *reinterpret_cast<volatile uint32_t*>(0xE000ED88u);
   arm_v7m_cpacr |= kFpuEnableMask;
+
+  // Ensure the FPU configuration is committed and enabled before continuing and
+  // potentially executing any FPU instructions, however rare that may be during
+  // startup.
+  asm volatile(
+      " dsb \n"
+      " isb \n"
+      // clang-format off
+      : /*output=*/
+      : /*input=*/
+      : /*clobbers=*/"memory"
+      // clang-format on
+  );
 #endif  // PW_ARMV7M_ENABLE_FPU
 }