linux-kernel - [PATCH v2] drm/i915: Distribute switch variables for initialization

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Message-ID: <202002201602.92CADF7D@keescook>
Date:   Thu, 20 Feb 2020 16:05:17 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2] drm/i915: Distribute switch variables for initialization

Variables declared in a switch statement before any case statements
cannot be automatically initialized with compiler instrumentation (as
they are not part of any execution flow). With GCC's proposed automatic
stack variable initialization feature, this triggers a warning (and they
don't get initialized). Clang's automatic stack variable initialization
(via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
doesn't initialize such variables[1]. Note that these warnings (or silent
skipping) happen before the dead-store elimination optimization phase,
so even when the automatic initializations are later elided in favor of
direct initializations, the warnings remain.

To avoid these problems, move such variables into the "case" where
they're used or lift them up into the main function body.

drivers/gpu/drm/i915/display/intel_display.c: In function ‘check_digital_port_conflicts’:
drivers/gpu/drm/i915/display/intel_display.c:12963:17: warning: statement will never be executed [-Wswitch-unreachable]
12963 |    unsigned int port_mask;
      |                 ^~~~~~~~~

drivers/gpu/drm/i915/intel_pm.c: In function ‘vlv_get_fifo_size’:
drivers/gpu/drm/i915/intel_pm.c:474:7: warning: statement will never be executed [-Wswitch-unreachable]
  474 |   u32 dsparb, dsparb2, dsparb3;
      |       ^~~~~~
drivers/gpu/drm/i915/intel_pm.c: In function ‘vlv_atomic_update_fifo’:
drivers/gpu/drm/i915/intel_pm.c:1997:7: warning: statement will never be executed [-Wswitch-unreachable]
 1997 |   u32 dsparb, dsparb2, dsparb3;
      |       ^~~~~~

[1] https://bugs.llvm.org/show_bug.cgi?id=44916

Signed-off-by: Kees Cook <keescook@...omium.org>
---
v2: remove port_mask entirely (Ville Syrjälä)
v1: https://lore.kernel.org/lkml/20200220062258.68854-1-keescook@chromium.org
---
 drivers/gpu/drm/i915/display/intel_display.c | 7 ++-----
 drivers/gpu/drm/i915/intel_pm.c              | 4 ++--
 2 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 064dd99bbc49..5f8c61932e82 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -12960,7 +12960,6 @@ static bool check_digital_port_conflicts(struct intel_atomic_state *state)
 		WARN_ON(!connector_state->crtc);
 
 		switch (encoder->type) {
-			unsigned int port_mask;
 		case INTEL_OUTPUT_DDI:
 			if (WARN_ON(!HAS_DDI(to_i915(dev))))
 				break;
@@ -12968,13 +12967,11 @@ static bool check_digital_port_conflicts(struct intel_atomic_state *state)
 		case INTEL_OUTPUT_DP:
 		case INTEL_OUTPUT_HDMI:
 		case INTEL_OUTPUT_EDP:
-			port_mask = 1 << encoder->port;
-
 			/* the same port mustn't appear more than once */
-			if (used_ports & port_mask)
+			if (used_ports & BIT(encoder->port))
 				ret = false;
 
-			used_ports |= port_mask;
+			used_ports |= BIT(encoder->port);
 			break;
 		case INTEL_OUTPUT_DP_MST:
 			used_mst_ports |=
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index bd2d30ecc030..17d8833787c4 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -469,9 +469,9 @@ static void vlv_get_fifo_size(struct intel_crtc_state *crtc_state)
 	struct vlv_fifo_state *fifo_state = &crtc_state->wm.vlv.fifo_state;
 	enum pipe pipe = crtc->pipe;
 	int sprite0_start, sprite1_start;
+	u32 dsparb, dsparb2, dsparb3;
 
 	switch (pipe) {
-		u32 dsparb, dsparb2, dsparb3;
 	case PIPE_A:
 		dsparb = I915_READ(DSPARB);
 		dsparb2 = I915_READ(DSPARB2);
@@ -1969,6 +1969,7 @@ static void vlv_atomic_update_fifo(struct intel_atomic_state *state,
 	const struct vlv_fifo_state *fifo_state =
 		&crtc_state->wm.vlv.fifo_state;
 	int sprite0_start, sprite1_start, fifo_size;
+	u32 dsparb, dsparb2, dsparb3;
 
 	if (!crtc_state->fifo_changed)
 		return;
@@ -1994,7 +1995,6 @@ static void vlv_atomic_update_fifo(struct intel_atomic_state *state,
 	spin_lock(&uncore->lock);
 
 	switch (crtc->pipe) {
-		u32 dsparb, dsparb2, dsparb3;
 	case PIPE_A:
 		dsparb = intel_uncore_read_fw(uncore, DSPARB);
 		dsparb2 = intel_uncore_read_fw(uncore, DSPARB2);
-- 
2.20.1


-- 
Kees Cook