linux-kernel - [PATCH] [v2] drm/i915: use static const array for PICK macro

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Message-Id: <20171211124640.1010542-1-arnd@arndb.de>
Date:   Mon, 11 Dec 2017 13:46:22 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>, Imre Deak <imre.deak@...el.com>,
        Lionel Landwerlin <lionel.g.landwerlin@...el.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] [v2] drm/i915: use static const array for PICK macro

The varargs macro trick in _PIPE3/_PHY3/_PORT3 was meant as an optimization
to shrink the i915 kernel module by around 1000 bytes. However, the
downside is a size regression with CONFIG_KASAN, as I found from stack size
warnings with gcc-7.0.1:

before:
drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_get_hw_state':
drivers/gpu/drm/i915/intel_dpll_mgr.c:1644:1: error: the frame size of 176 bytes is larger than 100 bytes [-Werror=frame-larger-than=]
drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_enable':
drivers/gpu/drm/i915/intel_dpll_mgr.c:1548:1: error: the frame size of 224 bytes is larger than 100 bytes [-Werror=frame-larger-than=]

after:
drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_get_hw_state':
drivers/gpu/drm/i915/intel_dpll_mgr.c:1644:1: error: the frame size of 1016 bytes is larger than 1000 bytes [-Werror=frame-larger-than=]
drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_enable':
drivers/gpu/drm/i915/intel_dpll_mgr.c:1548:1: error: the frame size of 1960 bytes is larger than 1000 bytes [-Werror=frame-larger-than=]

I also checked the module sizes and got with gcc-7.0.1

original:
   text	   data	    bss	    dec	    hex	filename
2380830	1155436	   4448	3540714	 3606ea	drivers/gpu/drm/i915/i915-kasan.o
1298054	 543692	   2884	1844630	 1c2596	drivers/gpu/drm/i915/i915-nokasan.o

after ce64645d86ac:
   text	   data	    bss	    dec	    hex	filename
2389515	1154476	   4448	3548439	 362517	drivers/gpu/drm/i915/i915-kasan.o
1299639	 543692	   2884	1846215	 1c2bc7	drivers/gpu/drm/i915/i915-nokasan.o

with this patch:
   text	   data	    bss	    dec	    hex	filename
2381275	1163884	   4448	3549607	 3629a7	drivers/gpu/drm/i915/i915-kasan.o
1296038	 543692	   2884	1842614	 1c1db6	drivers/gpu/drm/i915/i915-nokasan.o

Actually showing a code size growth in .text both with and without kasan,
and my version gets most of it back at the expense of larger .data when
kasan is enabled.

Fixes: ce64645d86ac ("drm/i915: use variadic macros and arrays to choose port/pipe based registers")
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80114
Link: https://lkml.org/lkml/2017/3/20/1022
Cc: Jani Nikula <jani.nikula@...ux.intel.com>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
v2: rebased after a1986f4174a4 ("drm/i915: Remove unnecessary PORT3 definition.")
---
 drivers/gpu/drm/i915/i915_reg.h | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 09bf043c1c2e..36f4408503e1 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -139,7 +139,7 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
 	return !i915_mmio_reg_equal(reg, INVALID_MMIO_REG);
 }
 
-#define _PICK(__index, ...) (((const u32 []){ __VA_ARGS__ })[__index])
+#define _PICK(__index, ...) ({static const u32 __arr[] = { __VA_ARGS__ }; __arr[__index];})
 
 #define _PIPE(pipe, a, b) ((a) + (pipe)*((b)-(a)))
 #define _MMIO_PIPE(pipe, a, b) _MMIO(_PIPE(pipe, a, b))
@@ -3097,10 +3097,10 @@ enum i915_power_well_id {
 /*
  * Clock control & power management
  */
-#define _DPLL_A (dev_priv->info.display_mmio_offset + 0x6014)
-#define _DPLL_B (dev_priv->info.display_mmio_offset + 0x6018)
-#define _CHV_DPLL_C (dev_priv->info.display_mmio_offset + 0x6030)
-#define DPLL(pipe) _MMIO_PIPE3((pipe), _DPLL_A, _DPLL_B, _CHV_DPLL_C)
+#define _DPLL_A			0x6014
+#define _DPLL_B			0x6018
+#define _CHV_DPLL_C		0x6030
+#define DPLL(pipe) _MMIO(dev_priv->info.display_mmio_offset + _PICK((pipe), _DPLL_A, _DPLL_B, _CHV_DPLL_C))
 
 #define VGA0	_MMIO(0x6000)
 #define VGA1	_MMIO(0x6004)
@@ -3196,10 +3196,10 @@ enum i915_power_well_id {
 #define   SDVO_MULTIPLIER_SHIFT_HIRES		4
 #define   SDVO_MULTIPLIER_SHIFT_VGA		0
 
-#define _DPLL_A_MD (dev_priv->info.display_mmio_offset + 0x601c)
-#define _DPLL_B_MD (dev_priv->info.display_mmio_offset + 0x6020)
-#define _CHV_DPLL_C_MD (dev_priv->info.display_mmio_offset + 0x603c)
-#define DPLL_MD(pipe) _MMIO_PIPE3((pipe), _DPLL_A_MD, _DPLL_B_MD, _CHV_DPLL_C_MD)
+#define _DPLL_A_MD				0x601c
+#define _DPLL_B_MD				0x6020
+#define _CHV_DPLL_C_MD				0x603c
+#define DPLL_MD(pipe) _MMIO(dev_priv->info.display_mmio_offset + _PICK((pipe), _DPLL_A_MD, _DPLL_B_MD, _CHV_DPLL_C_MD))
 
 /*
  * UDI pixel divider, controlling how many pixels are stuffed into a packet.
-- 
2.9.0