linux-kernel - [PATCH AUTOSEL 5.7 138/274] drm/bridge: fix stack usage warning on old gcc

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Message-Id: <20200608230607.3361041-138-sashal@kernel.org>
Date:   Mon,  8 Jun 2020 19:03:51 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Arnd Bergmann <arnd@...db.de>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Sam Ravnborg <sam@...nborg.org>,
        Sasha Levin <sashal@...nel.org>,
        dri-devel@...ts.freedesktop.org
Subject: [PATCH AUTOSEL 5.7 138/274] drm/bridge: fix stack usage warning on old gcc

From: Arnd Bergmann <arnd@...db.de>

[ Upstream commit 78b0d99a68ecdc84728c99f4fef71942e9ecf35a ]

Some older versions of gcc badly optimize code that passes
an inline function argument into another function by reference,
causing huge stack usage:

drivers/gpu/drm/bridge/tc358768.c: In function 'tc358768_bridge_pre_enable':
drivers/gpu/drm/bridge/tc358768.c:840:1: error: the frame size of 2256 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]

Use a temporary variable as a workaround and add a comment pointing
to the gcc bug.

Fixes: ff1ca6397b1d ("drm/bridge: Add tc358768 driver")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@...com>
Signed-off-by: Sam Ravnborg <sam@...nborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428215408.4111675-1-arnd@arndb.de
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/gpu/drm/bridge/tc358768.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/tc358768.c b/drivers/gpu/drm/bridge/tc358768.c
index 1b39e8d37834..6650fe4cfc20 100644
--- a/drivers/gpu/drm/bridge/tc358768.c
+++ b/drivers/gpu/drm/bridge/tc358768.c
@@ -178,6 +178,8 @@ static int tc358768_clear_error(struct tc358768_priv *priv)
 
 static void tc358768_write(struct tc358768_priv *priv, u32 reg, u32 val)
 {
+	/* work around https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715 */
+	int tmpval = val;
 	size_t count = 2;
 
 	if (priv->error)
@@ -187,7 +189,7 @@ static void tc358768_write(struct tc358768_priv *priv, u32 reg, u32 val)
 	if (reg < 0x100 || reg >= 0x600)
 		count = 1;
 
-	priv->error = regmap_bulk_write(priv->regmap, reg, &val, count);
+	priv->error = regmap_bulk_write(priv->regmap, reg, &tmpval, count);
 }
 
 static void tc358768_read(struct tc358768_priv *priv, u32 reg, u32 *val)
-- 
2.25.1