linux-kernel - [PATCH] drm/i915: Convert _print

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Date:   Tue,  9 Oct 2018 10:14:01 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH] drm/i915: Convert _print_param to a macro

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.

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@...gle.com>
Signed-off-by: Nathan Chancellor <natechancellor@...il.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