Re: [XEN PATCH v7 42/51] build: grab common EFI source files in arch specific dir

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Thu, Oct 21, 2021 at 01:24:27PM +0200, Jan Beulich wrote:
> On 21.10.2021 13:03, Anthony PERARD wrote:
> > On Mon, Oct 18, 2021 at 10:48:26AM +0200, Jan Beulich wrote:
> >> On 15.10.2021 18:29, Anthony PERARD wrote:
> >>> On Thu, Oct 14, 2021 at 10:51:44AM +0200, Jan Beulich wrote:
> >>>> On 24.08.2021 12:50, Anthony PERARD wrote:
> >>>>>  obj-y += boot.init.o pe.init.o ebmalloc.o runtime.o
> >>>>>  obj-$(CONFIG_ACPI) +=  efi-dom0.init.o
> >>>>> +
> >>>>> +$(obj)/%.c: common/efi/%.c
> >>>>> +       $(Q)cp -f $< $@
> >>>>
> >>>> In case both trees are on the same file system, trying to hardlink first
> >>>> would seem desirable. When copying, I think you should also pass -p.
> >>>
> >>> I don't know if doing an hardlink is a good thing to do, I'm not sure of
> >>> the kind of issue this could bring. As for -p, I don't think it's a good
> >>> idea to copy the mode, ownership, and timestamps of the source file, I'd
> >>> rather have the timestamps that Make expect, e.i. "now".
> >>
> >> Why would "now" be correct (or expected) in any way? The cloned file is no
> >> different from the original. Nevertheless I agree that -p is not ideal;
> >> it's just that the more fine grained option to preserve just the timestamp
> >> is non-standard afaik. You could try that first and fall back to -p ...
> >> Otherwise, failing hard linking and using "cp -p", I'm afraid I'd prefer
> >> symlinking despite the arguments against it that you name in the
> >> description.
> > 
> > I guess I'm missing something, is there a reason to keep/copy the
> > timestamps of the original files?
> 
> Avoidance of confusion is my main aim here. I certainly would be puzzled
> to see what looks like a source file to have a time stamp much newer than
> expected.

So, there isn't really anything to do with the timestamps :-). I guess
we could keep using symbolic links, but force update the link at every
build.

I've tried that:
    $(obj)/%.c: $(abs_srctree)/common/efi/%.c FORCE
        $(Q)ln -nsf $< $@

and make seems happy. The link command run every time (due to adding
FORCE), but the `CC` command isn't, so that seems good. The recipe that
would run the `CC` command check if the prerequisite are newer than the
target using $? so it doesn't matter if the rule that update the source
file as run or not.

Thanks,

-- 
Anthony PERARD