Re: [Intel-gfx] [PATCH] drm/i915: Convert _print_param to a macro

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 10 Oct 2018, Nick Desaulniers <ndesaulniers@google.com> wrote:
> On Wed, Oct 10, 2018 at 1:30 PM Michal Wajdeczko
> <michal.wajdeczko@intel.com> wrote:
>>
>> On Wed, 10 Oct 2018 14:01:40 +0200, Jani Nikula
>> <jani.nikula@linux.intel.com> wrote:
>>
>> > On Tue, 09 Oct 2018, Nick Desaulniers <ndesaulniers@google.com> wrote:
>> >> On Tue, Oct 9, 2018 at 10:14 AM Nathan Chancellor
>> >> <natechancellor@gmail.com> wrote:
>> >>>
>> >>> When building the kernel with Clang with defconfig and CONFIG_64BIT
>> >>> disabled, vmlinux fails to link because of the BUILD_BUG in
>> >>> _print_param.
>> >>>
>> >>> ld: drivers/gpu/drm/i915/i915_params.o: in function `i915_params_dump':
>> >>> i915_params.c:(.text+0x56): undefined reference to
>> >>> `__compiletime_assert_191'
>> >>>
>> >>> This function is semantically invalid unless the code is first inlined
>> >>> then constant folded, which doesn't work for Clang because semantic
>> >>> analysis happens before optimization/inlining. Converting this function
>> >>> to a macro avoids this problem and allows Clang to properly remove the
>> >>> BUILD_BUG during optimization.
>> >>
>> >> Thanks Nathan for the patch.  To provide more context, Clang does
>> >> semantic analysis before optimization, where as GCC does these
>> >> together (IIUC).  So the above link error is from the naked
>> >> BUILD_BUG().  Clang can't evaluate the __builtin_strcmp's statically
>> >> until inlining has occurred, but that optimization happens after
>> >> semantic analysis.  To do the inlining before semantic analysis, we
>> >> MUST leverage the preprocessor, which runs before the compiler starts
>> >> doing semantic analysis.  I suspect this code is not valid for GCC
>> >> unless optimizations are enabled (the kernel only does compile with
>> >> optimizations turned on).  This change allows us to build this
>> >> translation unit with Clang.
>> >>
>> >> Acked-by: Nick Desaulniers <ndesaulniers@google.com>
>> >> (Note: this is the change I suggested, so not sure whether Acked-by or
>> >> Reviewed-by is more appropriate).
>> >
>> > *Sad trombone*
>> >
>> > I'd rather see us converting more macros to static inlines than the
>> > other way round.
>> >
>> > I'll let others chime in if they have any better ideas, otherwise I'll
>> > apply this one.
>>
>> Option 1: Just drop BUILD_BUG() from _print_param() function.
>
> I was also thinking of this.

So does this fix the issue?

diff --git a/drivers/gpu/drm/i915/i915_params.c b/drivers/gpu/drm/i915/i915_params.c
index bd6bd8879cab..8d71886b5f03 100644
--- a/drivers/gpu/drm/i915/i915_params.c
+++ b/drivers/gpu/drm/i915/i915_params.c
@@ -184,7 +184,8 @@ static __always_inline void _print_param(struct drm_printer *p,
 	else if (!__builtin_strcmp(type, "char *"))
 		drm_printf(p, "i915.%s=%s\n", name, *(const char **)x);
 	else
-		BUILD_BUG();
+		WARN_ONCE(1, "no printer defined for param type %s (i915.%s)\n",
+			  type, name);
 }
 
 /**

---

>
>>
>> Option 2: Use aliases instead of real types in param() macros.
>
> Will that affect other users of I915_PARAMS_FOR_EACH than _print_param?
>
> Either way, thanks for the help towards resolving this! We appreciate it!
>
>>
>> Aliases can be same as in linux/moduleparam.h (charp|int|uint|bool)
>> We can convert aliases back to real types but it will also allow
>> to construct proper names for dedicated functions - see [1]
>>
>> Michal
>>
>> [1] https://patchwork.freedesktop.org/patch/255928/

I can't find this on the list; was this sent just to patchwork or what?

BR,
Jani.


>>
>>
>> >
>> > BR,
>> > Jani.
>> >
>> >>
>> >>>
>> >>> The output of 'objdump -D' is identically before and after this change
>> >>> for GCC regardless of if CONFIG_64BIT is set and allows Clang to link
>> >>> the kernel successfully with or without CONFIG_64BIT set.
>> >>>
>> >>> Link: https://github.com/ClangBuiltLinux/linux/issues/191
>> >>> Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
>> >>> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
>> >>> ---
>> >>>  drivers/gpu/drm/i915/i915_params.c | 29 +++++++++++++----------------
>> >>>  1 file changed, 13 insertions(+), 16 deletions(-)
>> >>>
>> >>> diff --git a/drivers/gpu/drm/i915/i915_params.c
>> >>> b/drivers/gpu/drm/i915/i915_params.c
>> >>> index 295e981e4a39..a0f20b9b6f2d 100644
>> >>> --- a/drivers/gpu/drm/i915/i915_params.c
>> >>> +++ b/drivers/gpu/drm/i915/i915_params.c
>> >>> @@ -174,22 +174,19 @@ i915_param_named(enable_dpcd_backlight, bool,
>> >>> 0600,
>> >>>  i915_param_named(enable_gvt, bool, 0400,
>> >>>         "Enable support for Intel GVT-g graphics virtualization host
>> >>> support(default:false)");
>> >>>
>> >>> -static __always_inline void _print_param(struct drm_printer *p,
>> >>> -                                        const char *name,
>> >>> -                                        const char *type,
>> >>> -                                        const void *x)
>> >>> -{
>> >>> -       if (!__builtin_strcmp(type, "bool"))
>> >>> -               drm_printf(p, "i915.%s=%s\n", name, yesno(*(const bool
>> >>> *)x));
>> >>> -       else if (!__builtin_strcmp(type, "int"))
>> >>> -               drm_printf(p, "i915.%s=%d\n", name, *(const int *)x);
>> >>> -       else if (!__builtin_strcmp(type, "unsigned int"))
>> >>> -               drm_printf(p, "i915.%s=%u\n", name, *(const unsigned
>> >>> int *)x);
>> >>> -       else if (!__builtin_strcmp(type, "char *"))
>> >>> -               drm_printf(p, "i915.%s=%s\n", name, *(const char **)x);
>> >>> -       else
>> >>> -               BUILD_BUG();
>> >>> -}
>> >>> +#define _print_param(p, name, type,
>> >>> x)                                        \
>> >>> +do
>> >>> {
>> >>> \
>> >>> +       if (!__builtin_strcmp(type,
>> >>> "bool"))                                   \
>> >>> +               drm_printf(p, "i915.%s=%s\n", name, yesno(*(const bool
>> >>> *)x));  \
>> >>> +       else if (!__builtin_strcmp(type,
>> >>> "int"))                               \
>> >>> +               drm_printf(p, "i915.%s=%d\n", name, *(const int
>> >>> *)x);          \
>> >>> +       else if (!__builtin_strcmp(type, "unsigned
>> >>> int"))                      \
>> >>> +               drm_printf(p, "i915.%s=%u\n", name, *(const unsigned
>> >>> int *)x); \
>> >>> +       else if (!__builtin_strcmp(type, "char
>> >>> *"))                            \
>> >>> +               drm_printf(p, "i915.%s=%s\n", name, *(const char
>> >>> **)x);        \
>> >>> +
>> >>> else
>> >>> \
>> >>> +
>> >>> BUILD_BUG();                                                   \
>> >>> +} while (0)
>> >>>
>> >>>  /**
>> >>>   * i915_params_dump - dump i915 modparams
>> >>> --
>> >>> 2.19.0
>> >>>

-- 
Jani Nikula, Intel Open Source Graphics Center