Adding some info about stability of ARM boards

Patch by Mikael Lyngvig

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194794 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 26 insertions, 4 deletions

diff --git a/docs/HowToBuildOnARM.rst b/docs/HowToBuildOnARM.rst
index b58dfac..f2edaef 100644
--- a/docs/HowToBuildOnARM.rst
+++ b/docs/HowToBuildOnARM.rst
@@ -25,15 +25,15 @@ on the ARMv6 and ARMv7 architectures and may be inapplicable to older chips.
    process will very likely fail due to insufficient memory. In any
    case it is probably a good idea to set up a swap partition.
 
-#. If you want to run ``make
-   check-all`` after building LLVM/Clang, to avoid false alarms (eg, ARCMT
-   failure) please use at least the following configuration:
+#. If you want to run ``make check-all`` after building LLVM/Clang, to avoid
+   false alarms (e.g., ARCMT failure) please use at least the following
+   configuration:
 
    .. code-block:: bash
 
      $ ../$LLVM_SRC_DIR/configure --with-abi=aapcs-vfp
 
-#. The most popular linaro/ubuntu OS's for ARM boards, eg, the
+#. The most popular Linaro/Ubuntu OS's for ARM boards, e.g., the
    Pandaboard, have become hard-float platforms. The following set
    of configuration options appears to be a good choice for this
    platform:
@@ -45,3 +45,25 @@ on the ARMv6 and ARMv7 architectures and may be inapplicable to older chips.
      --target=armv7l-unknown-linux-gnueabihf --with-cpu=cortex-a9 \
      --with-float=hard --with-abi=aapcs-vfp --with-fpu=neon \
      --enable-targets=arm --enable-optimized --enable-assertions
+
+#. ARM development boards can be unstable and you may experience that cores
+   are disappearing, caches being flushed on every big.LITTLE switch, and
+   other similar issues.  To help ease the effect of this, set the Linux
+   scheduler to "performance" on **all** cores using this little script:
+
+   .. code-block:: bash
+
+      # The code below requires the package 'cpufrequtils' to be installed.
+      for ((cpu=0; cpu<`grep -c proc /proc/cpuinfo`; cpu++)); do
+          sudo cpufreq-set -c $cpu -g performance
+      done
+
+#. Running the build on SD cards is ok, but they are more prone to failures
+   than good quality USB sticks, and those are more prone to failures than
+   external hard-drives (those are also a lot faster). So, at least, you
+   should consider to buy a fast USB stick.  On systems with a fast eMMC,
+   that's a good option too.
+
+#. Make sure you have a decent power supply (dozens of dollars worth) that can
+   provide *at least* 4 amperes, this is especially important if you use USB
+   devices with your board.