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

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Date:   Wed, 29 Apr 2020 10:56:19 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Arnd Bergmann <arnd@...db.de>, Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Peter Ujfalusi <peter.ujfalusi@...com>
CC:     Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Sebastian Reichel <sebastian.reichel@...labora.com>,
        <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/bridge: fix stack usage warning on old gcc

On 29/04/2020 00:53, 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@...db.de>
> ---
>   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)
> 

tc358768_write is not inline. What is the inline function here? Or is tc358768_write optimized to 
inline by the compiler?

  Tomi

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