On Thu, 2019-08-01 at 11:50 +0900, Masahiro Yamada wrote: > On Thu, Aug 1, 2019 at 4:04 AM Rikard Falkeborn > <rikard.falkeb...@gmail.com> wrote: > > GENMASK() and GENMASK_ULL() are supposed to be called with the high bit > > as the first argument and the low bit as the second argument. Mixing > > them will return a mask with zero bits set. > > > > Recent commits show getting this wrong is not uncommon, see e.g. > > commit aa4c0c9091b0 ("net: stmmac: Fix misuses of GENMASK macro") and > > commit 9bdd7bb3a844 ("clocksource/drivers/npcm: Fix misuse of GENMASK > > macro"). > > > > To prevent such mistakes from appearing again, add compile time sanity > > checking to the arguments of GENMASK() and GENMASK_ULL(). If both the > > arguments are known at compile time, and the low bit is higher than the > > high bit, break the build to detect the mistake immediately. > > > > Since GENMASK() is used in declarations, BUILD_BUG_OR_ZERO() must be > > used instead of BUILD_BUG_ON(), and __is_constexpr() must be used instead > > of __builtin_constant_p(). > > > > Commit 95b980d62d52 ("linux/bits.h: make BIT(), GENMASK(), and friends > > available in assembly") made the macros in linux/bits.h available in > > assembly. Since neither BUILD_BUG_OR_ZERO() or __is_constexpr() are asm > > compatible, disable the checks if the file is included in an asm file. > > > > Signed-off-by: Rikard Falkeborn <rikard.falkeb...@gmail.com> > > --- > > Joe Perches sent a series to fix the existing misuses of GENMASK() that > > needs to be merged before this to avoid build failures. Currently, 7 of > > the patches were not in Linus tree, and 2 were not in linux-next. > > > > Also, there's currently no asm users of bits.h, but since it was made > > asm-compatible just two weeks ago it would be a shame to break it right > > away... [] > > diff --git a/include/linux/bits.h b/include/linux/bits.h [] > > #define GENMASK(h, l) \ > > + (GENMASK_INPUT_CHECK(h, l) + \ > > (((~UL(0)) - (UL(1) << (l)) + 1) & \ > > - (~UL(0) >> (BITS_PER_LONG - 1 - (h)))) > > + (~UL(0) >> (BITS_PER_LONG - 1 - (h))))) > > > > #define GENMASK_ULL(h, l) \ > > + (GENMASK_INPUT_CHECK(h, l) + \ > > (((~ULL(0)) - (ULL(1) << (l)) + 1) & \ > > - (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h)))) > > + (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h))))) > > This is getting cluttered with so many parentheses. > > One way of clean-up is to rename the current macros as follows: > > GENMASK() -> __GENMASK() > GENMASK_UL() -> __GENMASK_ULL() > > Then, > > #define GENMASK(h, l) (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > #define GENMASK_ULL(h, l) (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
Much nicer. It may be better still to use avoid multiple dereferences of each argument. Also it'd be useful to rename h and l to something like high_bit and low_bit or high and low.