From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756393AbdJJPHG (ORCPT ); Tue, 10 Oct 2017 11:07:06 -0400 Received: from mail-qt0-f174.google.com ([209.85.216.174]:47222 "EHLO mail-qt0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756176AbdJJPHC (ORCPT ); Tue, 10 Oct 2017 11:07:02 -0400 X-Google-Smtp-Source: AOwi7QD6Mqe7FKGXyeHGpunLDRLzxBrmJzAdgZa6Dc3WDWRVbRobN6gVju+jtgCWxtnMtc1RLV0yrA== Date: Tue, 10 Oct 2017 08:06:56 -0700 From: Jakub Kicinski To: Geert Uytterhoeven Cc: Geert Uytterhoeven , Arnd Bergmann , Andrew Morton , kbuild test robot , Linux-Arch , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] bitfield: Use __ffs64(x) to fix missing __ffsdi2() Message-ID: <20171010080656.1d12d6a7@cakuba.netronome.com> In-Reply-To: References: <1507538449-22775-1-git-send-email-geert+renesas@glider.be> <20171009155311.17e1c7f8@cakuba.netronome.com> Organization: Netronome Systems, Ltd. MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > 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 > >> Fixes: 3e9b3112ec74f192 ("add basic register-field manipulation macros") > >> Signed-off-by: Geert Uytterhoeven > >> --- > >> 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 > >> #include > >> > >> /* > >> @@ -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...