In user space, the trace information will show up under /debug/root/name, where debug is the debugfs mount point, and root and name are the directory names passed to trace_setup(). The file state can be read to get the current tracing state; an application can write start or stop to this file to turn tracing on or off. The file trace0 is the relay channel where tracing data can be read; on SMP systems with per-CPU channels there will be additional files (trace1...) for additional processors. The file dropped can be read to see how many trace records (if any) have been dropped due to buffer-full conditions.
All told, it is not a particularly complicated bit of code. Perhaps the most significant feature of this patch is that it is part of the infrastructure created and used by the SystemTap project. Getting this code into the mainline will make it that much easier for distributors to provide well-supported tracing facilities to their users. And that, in turn, should make users happy and give analysts one less thing to complain about.