linux-kernel - [PATCH 3/3] [RFC] Revert "drm/i915: use variadic macros and arrays to choose port/pipe based registers"

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Date:   Mon, 20 Mar 2017 10:40:29 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Daniel Vetter <daniel.vetter@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Mika Kuoppala <mika.kuoppala@...ux.intel.com>,
        Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Imre Deak <imre.deak@...el.com>,
        Ander Conselvan de Oliveira 
        <ander.conselvan.de.oliveira@...el.com>,
        Robert Bragg <robert@...bynine.org>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 3/3] [RFC] Revert "drm/i915: use variadic macros and arrays to choose port/pipe based registers"

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

before:
   text	   data	    bss	    dec	    hex	filename
2704592	 412025	  11104	3127721	 2fb9a9	drivers/gpu/drm/i915/i915.o

after:
   text	   data	    bss	    dec	    hex	filename
2718538	 412025	  11104	3141667	 2ff023	drivers/gpu/drm/i915/i915.o

showing a significant code size growth. This reverts the patch to avoid
the warning and get back to the better code with CONFIG_KASAN. If someone
has another idea to avoid the warning while keeping the more efficient
code for the non-KASAN case, that would obviously be better.

Fixes: ce64645d86ac ("drm/i915: use variadic macros and arrays to choose port/pipe based registers")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/gpu/drm/i915/i915_reg.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 04c8f69fcc62..aa732eccdeb5 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -48,8 +48,6 @@ 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 _PIPE(pipe, a, b) ((a) + (pipe)*((b)-(a)))
 #define _MMIO_PIPE(pipe, a, b) _MMIO(_PIPE(pipe, a, b))
 #define _PLANE(plane, a, b) _PIPE(plane, a, b)
@@ -58,11 +56,14 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
 #define _MMIO_TRANS(tran, a, b) _MMIO(_TRANS(tran, a, b))
 #define _PORT(port, a, b) ((a) + (port)*((b)-(a)))
 #define _MMIO_PORT(port, a, b) _MMIO(_PORT(port, a, b))
-#define _PIPE3(pipe, ...) _PICK(pipe, __VA_ARGS__)
+#define _PIPE3(pipe, a, b, c) ((pipe) == PIPE_A ? (a) : \
+			       (pipe) == PIPE_B ? (b) : (c))
 #define _MMIO_PIPE3(pipe, a, b, c) _MMIO(_PIPE3(pipe, a, b, c))
-#define _PORT3(port, ...) _PICK(port, __VA_ARGS__)
+#define _PORT3(port, a, b, c) ((port) == PORT_A ? (a) : \
+			       (port) == PORT_B ? (b) : (c))
 #define _MMIO_PORT3(pipe, a, b, c) _MMIO(_PORT3(pipe, a, b, c))
-#define _PHY3(phy, ...) _PICK(phy, __VA_ARGS__)
+#define _PHY3(phy, a, b, c) ((phy) == DPIO_PHY0 ? (a) : \
+			     (phy) == DPIO_PHY1 ? (b) : (c))
 #define _MMIO_PHY3(phy, a, b, c) _MMIO(_PHY3(phy, a, b, c))
 
 #define _MASKED_FIELD(mask, value) ({					   \
-- 
2.9.0