On Wed, Dec 23, 2009 at 01:48:09PM -0800, Linus Torvalds wrote:
>
>
> On Wed, 23 Dec 2009, Greg KH wrote:
> >
> > They do:
> > - reorg the device ids in one usb-serial driver. I took this
> > now as it's a merge pressure point, and this is just a code
> > move now, no functionality changed at all. It's odd that git
> > didn't show this as a code move in the diffstat, I used:
> > git diff -m --stat --summary
> > to generate the diffstat. Should I be doing something else?
> > Or is it because the code is being removed from one file and
> > placed in another one?
>
> Since the old file stays around, git doesn't consider it a rename.
>
> Now, you can actually change that a bit. You can use -C instead of -M,
> which enables copy-detection rather than just renames, and then it changes
> from
>
> drivers/usb/serial/ftdi_sio.c | 1 +
> drivers/usb/serial/ftdi_sio.h | 959 +------------------------------
> drivers/usb/serial/ftdi_sio_ids.h | 986 +++++++++++++++++++++++++++++++
>
> to
>
> drivers/usb/serial/ftdi_sio.c | 1 +
> drivers/usb/serial/ftdi_sio.h | 959 +-----------
> drivers/usb/serial/{ftdi_sio.h => ftdi_sio_ids.h} | 1785 ++++++++-------------
> copy drivers/usb/serial/{ftdi_sio.h => ftdi_sio_ids.h} (65%)
>
> which isn't actually any better (since while it notices the copy of the
> data, it _removes_ all the code whily copying, so the total number of
> lines actually goes up!).
>
> So -M doesn't really "help" in that the diff itself does end up larger,
> but at the same time it is somewhat informative in the sense that you do
> see where the data comes from.
>
> To see another facet of it all, you could also use "-B" to say that you
> want to Break all old name associations if a file has changed a lot, and
> then enable rename (or copy) detection, and get
>
> drivers/usb/serial/ftdi_sio.c | 1 +
> drivers/usb/serial/ftdi_sio.h | 2063 ++++++---------------
> drivers/usb/serial/{ftdi_sio.h => ftdi_sio_ids.h} | 1785 +++++++------------
> rewrite drivers/usb/serial/ftdi_sio.h (68%)
> rename drivers/usb/serial/{ftdi_sio.h => ftdi_sio_ids.h} (65%)
>
> which is the largest of all diffs but it shows another side of the
> situation: that sio.h file has is now a "rewrite", and the end result is
> substantially different from the original. So git shows it as a "rename in
> place" where the diff shows up as a total delete followed by a total
> addition.
>
> As you can see, git does see that you moved data around, but since a lot
> of the data also _stayed_ in the original file, the copy/break operations
> really don't help. You cannot show "copy this part of this file into that
> other file" as a diff.
>
> Btw, we _do_ do the "copy this part of this file into that other file"
> when you do "git blame -C", so you can now do
>
> git blame -C drivers/usb/serial/ftdi_sio_ids.h
>
> and it will follow the history of the lines in that file back to the
> ftdi_sio.h file. So the fact that git doesn't show it in the diffs is
> really because diffs always work on a whole-file basis (ie you can show a
> "rename file" diff, but you can't show a "move one function or a set of
> ID's from one file to another" diff.
Thanks for the through explaination. I guess I'll just stick with '-M'
on the diffstat summaries for now, but it is good to know that git can
track this type of change through the changeset, which is the most
important thing.
thanks,
greg k-h
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