linux-kernel - Re: [PATCH] bitfield: Use __ffs64(x) to fix missing _

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Date:   Tue, 10 Oct 2017 08:06:56 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kbuild test robot <fengguang.wu@...el.com>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bitfield: Use __ffs64(x) to fix missing __ffsdi2()

On Tue, 10 Oct 2017 09:03:50 +0200, Geert Uytterhoeven wrote:
> Hi Jakub,
> 
> On Tue, Oct 10, 2017 at 12:53 AM, Jakub Kicinski
> <jakub.kicinski@...ronome.com> wrote:
> > On Mon,  9 Oct 2017 10:40:49 +0200, Geert Uytterhoeven wrote:  
> >> On most architectures[*], gcc turns __builtin_ffsll() into a call to
> >> __ffsdi2(), which is not provided by any architecture, leading to
> >> failures like:
> >>
> >>     rcar-gen3-cpg.c:(.text+0x289): undefined reference to `__ffsdi2'
> >>
> >> To fix this, use __ffs64() instead, which is available on all
> >> architectures.
> >>
> >> [*] Known exceptions are some 64-bit architectures like amd64, arm64,
> >>     ia64, powerpc64, and tilegx.
> >>
> >> Reported-by: kbuild test robot <fengguang.wu@...el.com>
> >> Fixes: 3e9b3112ec74f192 ("add basic register-field manipulation macros")
> >> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> >> ---
> >>  include/linux/bitfield.h | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> >> index 8b9d6fff002db113..0a827677372756fa 100644
> >> --- a/include/linux/bitfield.h
> >> +++ b/include/linux/bitfield.h
> >> @@ -15,6 +15,7 @@
> >>  #ifndef _LINUX_BITFIELD_H
> >>  #define _LINUX_BITFIELD_H
> >>
> >> +#include <linux/bitops.h>
> >>  #include <linux/bug.h>
> >>
> >>  /*
> >> @@ -46,7 +47,7 @@
> >>   *  reg |= FIELD_PREP(REG_FIELD_C, c);
> >>   */
> >>
> >> -#define __bf_shf(x) (__builtin_ffsll(x) - 1)
> >> +#define __bf_shf(x) __ffs64(x)  
> >
> > Hm.  The build bot failure made me think.  I think rcar-gen3-cpg.c may
> > be doing something wrong here, could you point me at the patch in
> > question?  I don't see any FIELD_* there in Linus's tree.  
> 
> See series "[PATCH v3 0/6] clk: renesas: r8a779[56]: Add Z and Z2 clock
> support" (https://www.spinics.net/lists/arm-kernel/msg609499.html).
> 
> > __bf_shf() is supposed to be used with constant masks only, therefore
> > the call must be optimized away completely.
> >  
> >>  #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)                    \
> >>       ({                                                              \  
> 
> IC.
> 
> Yes, it looks like __ffs64() can't be optimized away like __builtin_ffsll() :-(
> 
> Apparently the patch series above uses __bf_shf() directly, to avoid the
> BUILD_BUG_ON_MSG(), which doesn't work when the call isn't optimized away.
> Sorry for not noticing that before...
> 
> One way to fix that (non-)API abuse would be to get rid of __bf_shf(),
> and open code it as __builtin_ffsll(x) - 1 everywhere...
> 
> What do you think?

I'm starting to feel like we/I should come up with a way to convert the
masks into something that can be passed to functions :(  The "mask must
be constant" problem is coming up again and again.  The only problem is
I'm not sure how to pack the mask+shift into a single argument :(  Also
the mask can be 32 or 64 bit, it would be nice to not force the output
to always be 64bit...