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

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On Wed, 10 Oct 2018 14:01:40 +0200, Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote:

On Tue, 09 Oct 2018, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
On Tue, Oct 9, 2018 at 10:14 AM Nathan Chancellor
<natechancellor@xxxxxxxxx> 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@xxxxxxxxxx>
(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.

Option 2: Use aliases instead of real types in param() macros.

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/



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@xxxxxxxxxx>
Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
---
 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

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