Hi, On 3/26/24 03:08, Vincent Donnefort wrote: > It is now possible to mmap() a ring-buffer to stream its content. Add > some documentation and a code example. > > Signed-off-by: Vincent Donnefort <vdonnef...@google.com> >
> diff --git a/Documentation/trace/ring-buffer-map.rst > b/Documentation/trace/ring-buffer-map.rst > new file mode 100644 > index 000000000000..0426ab4bcf3d > --- /dev/null > +++ b/Documentation/trace/ring-buffer-map.rst > @@ -0,0 +1,106 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +================================== > +Tracefs ring-buffer memory mapping > +================================== > + > +:Author: Vincent Donnefort <vdonnef...@google.com> > + > +Overview > +======== > +Tracefs ring-buffer memory map provides an efficient method to stream data > +as no memory copy is necessary. The application mapping the ring-buffer > becomes > +then a consumer for that ring-buffer, in a similar fashion to trace_pipe. > + > +Memory mapping setup > +==================== > +The mapping works with a mmap() of the trace_pipe_raw interface. > + > +The first system page of the mapping contains ring-buffer statistics and > +description. It is referred as the meta-page. One of the most important > field of referred to as fields > +the meta-page is the reader. It contains the sub-buffer ID which can be > safely > +read by the mapper (see ring-buffer-design.rst). > + > +The meta-page is followed by all the sub-buffers, ordered by ascendant ID. > It is ascending might be better IMO. > +therefore effortless to know where the reader starts in the mapping: > + > +.. code-block:: c > + > + reader_id = meta->reader->id; > + reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size; > + > +When the application is done with the current reader, it can get a new one > using > +the trace_pipe_raw ioctl() TRACE_MMAP_IOCTL_GET_READER. This ioctl also > updates > +the meta-page fields. [snip] Thanks. -- #Randy