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

Lucas De Marchi
2024-01-18T23:25:06Z

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))
+
#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: Before=2980529, After=2980529, chg +0.00%

thanks
Lucas De Marchi

>
>Thanks,
>Yury