The Impact line is not a replacement for a change log, it is simply the reason for the patch.
Well, that is the way I prefer to think of it. When Ingo asked me to start including Impact lines, I struggled with it. I'm guilty of doing the
that Linus complained about. Because, really, that is useless. But trying to come up with an "Impact" for the patch I found very difficult. But now I think of it more as the reason for the patch, not what the patch actually does. This is the typical changelog I submit:
Detail description of the what the patch does.
[ Impact: what is the purpose of the patch. ]
Here's an example:
ring-buffer: only warn on wrap if buffer is bigger than two pages
On boot up, to save memory, ftrace allocates the minimum buffer
which is two pages. Ftrace also goes through a series of tests
(when configured) on boot up. These tests can fill up a page within
a single interrupt.
The ring buffer also has a WARN_ON when it detects that the buffer was
completely filled within a single commit (other commits are allowed to
Combine the small buffer on start up, with the tests that can fill more
than a single page within an interrupt, this can trigger the WARN_ON.
This patch makes the WARN_ON only happen when the ring buffer consists
of more than two pages.
[ Impact: prevent false WARN_ON in ftrace startup tests ]
The subject and description describe what the patch does. The description may even go into detail of the history that caused this patch. But the Impact line is simply a one liner summary of the purpose of the patch.