On 06/03/2025 at 00:47, Yury Norov wrote: > On Wed, Mar 05, 2025 at 10:00:15PM +0900, Vincent Mailhol via B4 Relay wrote: >> From: Yury Norov <yury.norov@xxxxxxxxx> >> >> Add __GENMASK_t() which generalizes __GENMASK() to support different >> types, and implement fixed-types versions of GENMASK() based on it. >> The fixed-type version allows more strict checks to the min/max values >> accepted, which is useful for defining registers like implemented by >> i915 and xe drivers with their REG_GENMASK*() macros. >> >> The strict checks rely on shift-count-overflow compiler check to fail >> the build if a number outside of the range allowed is passed. >> Example: >> >> #define FOO_MASK GENMASK_U32(33, 4) >> >> will generate a warning like: >> >> ../include/linux/bits.h:41:31: error: left shift count >= width of type [-Werror=shift-count-overflow] >> 41 | (((t)~0ULL - ((t)(1) << (l)) + 1) & \ >> | ^~ >> >> Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx> >> Signed-off-by: Lucas De Marchi <lucas.demarchi@xxxxxxxxx> >> Acked-by: Jani Nikula <jani.nikula@xxxxxxxxx> >> Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx> > > Co-developed-by? OK. I will keep you as the main author and credit me as Co-developer. >> --- >> Changelog: >> >> v3 -> v4: >> >> - The v3 is one year old. Meanwhile people started using >> __GENMASK() directly. So instead of generalizing __GENMASK() to >> support different types, add a new GENMASK_t(). >> >> - replace ~0ULL by ~_ULL(0). Otherwise, __GENMASK_t() would fail >> in asm code. >> >> - Make GENMASK_U8() and GENMASK_U16() return an unsigned int. In >> v3, due to the integer promotion rules, these were returning a >> signed integer. By casting these to unsigned int, at least the > > This comment will disappear when I'll apply the patch. Can you comment > it in the code instead? Ack. I will add below comment in the code: /* * Because of the C integer promotion rules, the U8 and the U16 * variants would immediately become signed integers when used in * expressions. Cast them to unsigned int so that, at least, the * signedness is preserved. */ (unless if you prefer to go back to the u8 and u16 casts, c.f. below). >> signedness is kept. >> --- >> include/linux/bitops.h | 1 - >> include/linux/bits.h | 33 +++++++++++++++++++++++++++++---- >> 2 files changed, 29 insertions(+), 5 deletions(-) >> >> diff --git a/include/linux/bitops.h b/include/linux/bitops.h >> index c1cb53cf2f0f8662ed3e324578f74330e63f935d..9be2d50da09a417966b3d11c84092bb2f4cd0bef 100644 >> --- a/include/linux/bitops.h >> +++ b/include/linux/bitops.h >> @@ -8,7 +8,6 @@ >> >> #include <uapi/linux/kernel.h> >> >> -#define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE) >> #define BITS_TO_LONGS(nr) __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(long)) >> #define BITS_TO_U64(nr) __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u64)) >> #define BITS_TO_U32(nr) __KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u32)) >> diff --git a/include/linux/bits.h b/include/linux/bits.h >> index 5f68980a1b98d771426872c74d7b5c0f79e5e802..f202e46d2f4b7899c16d975120f3fa3ae41556ae 100644 >> --- a/include/linux/bits.h >> +++ b/include/linux/bits.h >> @@ -12,6 +12,7 @@ >> #define BIT_ULL_MASK(nr) (ULL(1) << ((nr) % BITS_PER_LONG_LONG)) >> #define BIT_ULL_WORD(nr) ((nr) / BITS_PER_LONG_LONG) >> #define BITS_PER_BYTE 8 >> +#define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE) >> >> /* >> * Create a contiguous bitmask starting at bit position @l and ending at >> @@ -25,14 +26,38 @@ >> >> #define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(const_true((l) > (h))) >> >> -#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)) >> +/* >> + * Generate a mask for the specified type @t. Additional checks are made to >> + * guarantee the value returned fits in that type, relying on >> + * shift-count-overflow compiler check to detect incompatible arguments. >> + * For example, all these create build errors or warnings: >> + * >> + * - GENMASK(15, 20): wrong argument order >> + * - GENMASK(72, 15): doesn't fit unsigned long >> + * - GENMASK_U32(33, 15): doesn't fit in a u32 >> + */ >> +#define GENMASK_t(t, h, l) \ > > Agree with Andy. This should be GENMASK_TYPE, or triple-underscored > ___GENMASK() maybe. This _t thing looks misleading. My preference goes to GENMASK_TYPE(). >> + (GENMASK_INPUT_CHECK(h, l) + \ >> + (((t)~ULL(0) - ((t)1 << (l)) + 1) & \ >> + ((t)~ULL(0) >> (BITS_PER_TYPE(t) - 1 - (h))))) > > Can you rebase it on top of -next? In this dev cycle I merge a patch > that reverts the __GENMASK() back to: Oh, I did not realize that. Do you mean a rebase on top of: https://github.com/norov/linux/tree/bitmap-for-next ? I will do so. > #define __GENMASK(h, l) (((~_UL(0)) << (l)) & (~_UL(0) >> (BITS_PER_LONG - 1 - (h)))) > >> +#define GENMASK(h, l) GENMASK_t(unsigned long, h, l) >> +#define GENMASK_ULL(h, l) GENMASK_t(unsigned long long, h, l) > > This makes __GENMASK() and __GENMASK_ULL() unused in the kernel, other > than in uapi. Or I misunderstand it? Correct. > Having, in fact, different implementations of the same macro for kernel > and userspace is a source of problems. Can we move GENMASK_TYPE() to uapi, > and implement __GENMASK() on top of them? If not, I'd prefer to keep > GENMASK and GENMASK_ULL untouched. This is something which I tried to explain in the cover letter. I am not confident to declare GENMASK_TYPE() in the uapi and expose it to the userland. If we do so, any future change in the parameters would be a user breaking change. __GENMASK_U128() looks already too much to me for the uapi, I am not keen to bloat it even more with GENMASK_TYPE(). This plus the fact that if we use GENMASK_TYPE() to generate the asm variant, then we can not rely on sizeof() in the definition which makes everything over complicated. I acknowledge that not having a common denominator is not best, but I see this as an acceptable tradeoff. > Can you run bloat-o-meter and ensure there's no unwanted effects on > code generation? Ack, but that will be tomorrow :) >> /* >> * Missing asm support >> * >> + * __GENMASK_U*() depends on BITS_PER_TYPE() which would not work in the asm > > And there's no __GENMASK_U*(), right? Yes, silly typo, sorry. Will fix in v5. >> + * code as BITS_PER_TYPE() relies on sizeof(), something not available in >> + * asm. Nethertheless, the concept of fixed width integers is a C thing which >> + * does not apply to assembly code. >> + */ >> +#define GENMASK_U8(h, l) ((unsigned int)GENMASK_t(u8, h, l)) >> +#define GENMASK_U16(h, l) ((unsigned int)GENMASK_t(u16, h, l)) > > Typecast to the type that user provides explicitly? And maybe do > in GENMASK_TYPE() I have a slight preference for the cast to unsigned int for the reason explained above. But that is not a deal breaker. If you believe that the u8/u16 casts are better, let me know, I will be happy to change it :) >> +#define GENMASK_U32(h, l) GENMASK_t(u32, h, l) >> +#define GENMASK_U64(h, l) GENMASK_t(u64, h, l) > > OK, this looks good. But GENMASK_U128() becomes a special case now. > The 128-bit GENMASK is unsued, but it's exported in uapi. Is there any > simple way to end up with a common implementation for all fixed-type > GENMASKs? What bothers me is that the 128 bit types are not something available on all architectures, c.f. the CONFIG_ARCH_SUPPORTS_INT128. So, I would need a U128() equivalent to the ULL() but which does not break on architectures which do not support 128 bits integers. This is where I am stuck. If someone can guide me on how to write a robust U128() macro, then I think the common implementation could be feasible. >> + >> +/* >> * __GENMASK_U128() depends on _BIT128() which would not work >> * in the asm code, as it shifts an 'unsigned __int128' data >> * type instead of direct representation of 128 bit constants > > This comment is duplicated by the previous one. Maybe just join them? > (Let's wait for a while for updates regarding GENMASK_U128 status before > doing it.) OK. I will wait for this one. I will probably send the v5 before we get the answer but I do not this this is an issue if we have two parallel streams. Yours sincerely, Vincent Mailhol
