Re: [PATCH] drm/bridge: fix stack usage warning on old gcc

From: Sam Ravnborg <sam@ravnborg.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Andrzej Hajda <a.hajda@samsung.com>,
	Neil Armstrong <narmstrong@baylibre.com>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	Peter Ujfalusi <peter.ujfalusi@ti.com>,
	Jernej Skrabec <jernej.skrabec@siol.net>,
	Jonas Karlman <jonas@kwiboo.se>,
	Sebastian Reichel <sebastian.reichel@collabora.com>,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	Tomi Valkeinen <tomi.valkeinen@ti.com>,
	Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
	Boris Brezillon <boris.brezillon@collabora.com>
Subject: Re: [PATCH] drm/bridge: fix stack usage warning on old gcc
Date: Wed, 29 Apr 2020 22:46:13 +0200	[thread overview]
Message-ID: <20200429204613.GA24103@ravnborg.org> (raw)
In-Reply-To: <20200428215408.4111675-1-arnd@arndb.de>

Hi Arnd.

On Tue, Apr 28, 2020 at 11:53:54PM +0200, Arnd Bergmann wrote:
> 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@arndb.de>

Thanks, pushed to drm-misc-next with Tomi's r-b.

	Sam

> ---
>  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)
> -- 
> 2.26.0
> 
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