Re: Re: Re: [Intel-xe] [PATCH 2/3] linux/bits.h: Add fixed-width GENMASK and BIT macros

Yury Norov
2024-01-19T02:02:00Z

On Thu, Jan 18, 2024 at 05:25:00PM -0600, Lucas De Marchi wrote:
> SA2PR11MB4874
> X-OriginatorOrg: intel.com
> Status: RO
> Content-Length: 6257
> Lines: 150
>
> On Thu, Jan 18, 2024 at 01:48:43PM -0800, Yury Norov wrote:
> > On Thu, Jan 18, 2024 at 02:42:12PM -0600, Lucas De Marchi wrote:
> > > Hi,
> > >
> > > Reviving this thread as now with xe driver merged we have 2 users for
> > > a fixed-width BIT/GENMASK.
> >
> > Can you point where and why?
>
> See users of REG_GENMASK and REG_BIT in drivers/gpu/drm/i915 and
> drivers/gpu/drm/xe. I think the register definition in the xe shows it
> in a good way:
>
> drivers/gpu/drm/xe/regs/xe_gt_regs.h
>
> The GPU registers are mostly 32-bit wide. We don't want to accidently do
> something like below (s/30/33/ added for illustration purposes):
>
> #define LSC_CHICKEN_BIT_0 XE_REG_MCR(0xe7c8)
> #define DISABLE_D8_D16_COASLESCE REG_BIT(33)
>
> Same thing for GENMASK family of macros and for registers that are 16 or
> 8 bits. See e.g. drivers/gpu/drm/i915/display/intel_cx0_phy_regs.h
>
>
> >
> > > On Wed, Jun 21, 2023 at 07:20:59PM -0700, Yury Norov wrote:
> > > > Hi Lucas, all!
> > > >
> > > > (Thanks, Andy, for pointing to this thread.)
> > > >
> > > > On Mon, May 08, 2023 at 10:14:02PM -0700, Lucas De Marchi wrote:
> > > > > Add GENMASK_U32(), GENMASK_U16() and GENMASK_U8() macros to create
> > > > > masks for fixed-width types and also the corresponding BIT_U32(),
> > > > > BIT_U16() and BIT_U8().
> > > >
> > > > Can you split BIT() and GENMASK() material to separate patches?
> > > >
> > > > > All of those depend on a new "U" suffix added to the integer constant.
> > > > > Due to naming clashes it's better to call the macro U32. Since C doesn't
> > > > > have a proper suffix for short and char types, the U16 and U18 variants
> > > > > just use U32 with one additional check in the BIT_* macros to make
> > > > > sure the compiler gives an error when the those types overflow.
> > > >
> > > > I feel like I don't understand the sentence...
> > > >
> > > > > The BIT_U16() and BIT_U8() need the help of GENMASK_INPUT_CHECK(),
> > > > > as otherwise they would allow an invalid bit to be passed. Hence
> > > > > implement them in include/linux/bits.h rather than together with
> > > > > the other BIT* variants.
> > > >
> > > > I don't think it's a good way to go because BIT() belongs to a more basic
> > > > level than GENMASK(). Not mentioning possible header dependency issues.
> > > > If you need to test against tighter numeric region, I'd suggest to
> > > > do the same trick as GENMASK_INPUT_CHECK() does, but in uapi/linux/const.h
> > > > directly. Something like:
> > > > #define _U8(x) (CONST_GT(U8_MAX, x) + _AC(x, U))
> > >
> > > but then make uapi/linux/const.h include linux/build_bug.h?
> > > I was thinking about leaving BIT() define where it is, and add the
> > > fixed-width versions in this header. I was thinking uapi/linux/const.h
> > > was more about allowing the U/ULL suffixes for things shared with asm.
> >
> > You can't include kernel headers in uapi code. But you can try doing
> > vice-versa: implement or move the pieces you need to share to the
> > uapi/linux/const.h, and use them in the kernel code.
>
> but in this CONST_GE() should trigger a BUG/static_assert
> on U8_MAX < x. AFAICS that check can't be on the uapi/ side,
> so there's nothing much left to change in uapi/linux/const.h.
>
> I'd expect drivers to be the primary user of these fixed-width BIT
> variants, hence the proposal to do in include/linux/bits.h.
> Ssomething like this WIP/untested diff (on top of your previous patch):
>
>
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index cb94128171b2..409cd10f7597 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -24,12 +24,16 @@
> #define GENMASK_INPUT_CHECK(h, l) \
> (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> __is_constexpr((l) > (h)), (l) > (h), 0)))
> +#define BIT_INPUT_CHECK(type, b) \
> + ((BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> + __is_constexpr(b), (b) >= BITS_PER_TYPE(type), 0))))
> #else
> /*
> * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
> * disable the input check if that is the case.
> */
> #define GENMASK_INPUT_CHECK(h, l) 0
> +#define BIT_INPUT_CHECK(type, b) 0
> #endif
> #define __GENMASK(t, h, l) \
> @@ -44,4 +48,9 @@
> #define GENMASK_U32(h, l) __GENMASK(u32, h, l)
> #define GENMASK_U64(h, l) __GENMASK(u64, h, l)
> +#define BIT_U8(b) (u8)(BIT_INPUT_CHECK(u8, b) + BIT(b))
> +#define BIT_U16(b) (u16)(BIT_INPUT_CHECK(u16, b) + BIT(b))
> +#define BIT_U32(b) (u32)(BIT_INPUT_CHECK(u32, b) + BIT(b))
> +#define BIT_U64(b) (u64)(BIT_INPUT_CHECK(u64, b) + BIT(b))

Can you add some vertical spacing here, like between GENMASK and BIT
blocks?

> +
> #endif /* __LINUX_BITS_H */
>
> >
> > In the worst case, you can just implement the macro you need in the
> > uapi header, and make it working that way.
> >
> > Can you confirm that my proposal increases the kernel size? If so, is
> > there any way to fix it? If it doesn't, I'd prefer to use the
> > __GENMASK() approach.
>
> I agree on continuing with your approach. The bloat-o-meter indeed
> showed almost no difference. `size ....i915.o` on the other hand
> increased, but then decreased when I replaced our current REG_GENMASK()
> implementation to reuse the new GENMASK_U*()
>
> $ # test-genmask.00: before any change
> $ # test-genmask.01: after your patch to GENMASK
> $ # test-genmask.01: after converting drivers/gpu/drm/i915/i915_reg_defs.h
> to use the new macros
> $ size build64/drivers/gpu/drm/i915/i915.o-test-genmask.*
> text data bss dec hex filename
> 4506628 215083 7168 4728879 48282f build64/drivers/gpu/drm/i915/i915.o-test-genmask.00
> 4511084 215083 7168 4733335 483997 build64/drivers/gpu/drm/i915/i915.o-test-genmask.01
> 4493292 215083 7168 4715543 47f417 build64/drivers/gpu/drm/i915/i915.o-test-genmask.02
>
> $ ./scripts/bloat-o-meter build64/drivers/gpu/drm/i915/i915.o-test-genmask.0[01]
> add/remove: 0/0 grow/shrink: 2/1 up/down: 4/-5 (-1)
> Function old new delta
> intel_drrs_activate 399 402 +3
> intel_psr_invalidate 546 547 +1
> intel_psr_flush 880 875 -5
> Total: Before=2980530, After=2980529, chg -0.00%
>
> $ ./scripts/bloat-o-meter build64/drivers/gpu/drm/i915/i915.o-test-genmask.0[12]
> add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
> Function old new delta
> Total

OK then. With the above approach, fixed-type BIT() macros look like wrappers
around the plain BIT(), and I think, we can live with that.

Can you send all the material as a proper series, including my
GENMASK patch, your patch above and a patch that switches your driver
to using the new API? I'll take it then in bitmap-for-next when the
merge window will get closed.

Thanks,
Yury