docs/manual: add section about build time graphing

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/common-usage.txt

@@ -201,3 +201,50 @@ supported.
 --------------------------------
 GRAPH_OUT=svg make graph-depends
 --------------------------------
+
+Graphing the build duration
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+[[graph-duration]]
+
+When the build of a system takes a long time, it is sometimes useful
+to be able to understand which packages are the longest to build, to
+see if anything can be done to speed up the build. In order to help
+such build time analysis, Buildroot collects the build time of each
+step of each package, and allows to generate graphs from these data.
+
+To generate the build time graph after a build, run:
+
+----------------
+make graph-build
+----------------
+
+This will generate a set of files in +output/graphs+ :
+
+* +build.hist-build.pdf+, an histogram of the build time for each
+  package, ordered in the build order.
+
+* +build.hist-duration.pdf+, an histogram of the build time for each
+  package, ordered by duration (longest first)
+
+* +build.hist-name.pdf+, an histogram of the build time for each
+  package, order by package name.
+
+* +build.pie-packages.pdf+, a pie chart of the build time per package
+
+* +build.pie-steps.pdf+, a pie chart of the global time spent in each
+  step of the packages build process.
+
+This +graph-build+ target requires the Python Matplotlib and Numpy
+libraries to be installed (+python-matplotlib+ and +python-numpy+ on
+most distributions), and also the +argparse+ module if you're using a
+Python version older than 2.7 (+python-argparse+ on most
+distributions).
+
+By default, the output format for the graph is PDF, but a different
+format can be selected using the +GRAPH_OUT+ environment variable. The
+only other format supported is PNG:
+
+----------------
+GRAPH_OUT=png make graph-build
+----------------