SA2PR11MB4874
X-OriginatorOrg: intel.com
Status: RO
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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))
