blktrace: the ftrace interface needs CONFIG_TRACING

[Arnaldo Carvalho de Melo]

Impact: Fix Kconfig selection for blktrace

Also mention in the help text that blktrace now can be used using
the ftrace interface.

Cc: Jens Axboe <jens.axboe@xxxxxxxxxx>
Reported-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

diff --git a/block/Kconfig b/block/Kconfig
index 0cbb3b8..7cdaa1d 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -50,6 +50,8 @@ config BLK_DEV_IO_TRACE
 	select RELAY
 	select DEBUG_FS
 	select TRACEPOINTS
+	select TRACING
+	select STACKTRACE
 	help
 	  Say Y here if you want to be able to trace the block layer actions
 	  on a given queue. Tracing allows you to see any traffic happening
@@ -58,6 +60,12 @@ config BLK_DEV_IO_TRACE
 
 	  git://git.kernel.dk/blktrace.git
 
+	  Tracing also is possible using the ftrace interface, e.g.:
+
+	    echo 1 > /sys/block/sda/sda1/trace/enable
+	    echo blk > /sys/kernel/debug/tracing/current_tracer
+	    cat /sys/kernel/debug/tracing/trace_pipe
+
 	  If unsure, say N.
 
 config BLK_DEV_BSG
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