Re: [PATCH v3 1/8] xen: fix build when $(obj-y) consists of just blanks

[Anthony PERARD <anthony.perard@citrix.com>]

On Mon, Nov 23, 2020 at 04:20:52PM +0100, Jan Beulich wrote:
> This case can occur when combining empty lists
> 
> obj-y :=
> ...
> obj-y += $(empty)
> 
> or
> 
> obj-y := $(empty) $(empty)
> 
> where (only) blanks would accumulate. This was only a latent issue until
> now, but would become an active issue for Arm once lib/ gets populated
> with all respective objects going into the to be introduced lib.a.
> 
> Also address a related issue at this occasion: When an empty built_in.o
> gets created, .built_in.o.d will have its dependencies recorded. If, on
> a subsequent incremental build, an actual constituent of built_in.o
> appeared, the $(filter-out ) would leave these recorded dependencies in
> place. But of course the linker won't know what to do with C header
> files. (The apparent alternative of avoiding to pass $(c_flags) or
> $(a_flags) would not be reliable afaict, as among these flags there may
> be some affecting information conveyed via the object file to the
> linker. The linker, finding inconsistent flags across object files, may

How about using $(XEN_CFLAGS) instead of $(c_flags)? That should prevent
CC from generating the .*.o.d files while keeping the relevant flags. I
was planing to do that to avoid the issue, see:
https://lore.kernel.org/xen-devel/20200421161208.2429539-10-anthony.perard@citrix.com

> then error out.) Using just $(obj-y) won't work either: It breaks when
> the same object file is listed more than once.

Do we need to worry about having a object file been listed twice?
Wouldn't that be a mistake?

-- 
Anthony PERARD