linux-kernel - Re: [v2] drm: mediatek: change the variable type of rdma threshold

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Date:   Wed, 21 Jun 2017 14:14:56 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Bibby Hsieh <bibby.hsieh@...iatek.com>
Cc:     David Airlie <airlied@...ux.ie>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Cawa Cheng <cawa.cheng@...iatek.com>,
        Mao Huang <littlecvr@...omium.org>, CK Hu <ck.hu@...iatek.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        YT Shen <yt.shen@...iatek.com>,
        Yingjoe Chen <yingjoe.chen@...iatek.com>,
        Sascha Hauer <kernel@...gutronix.de>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [v2] drm: mediatek: change the variable type of rdma threshold

On Fri, May 19, 2017 at 05:57:23PM +0800, Bibby Hsieh wrote:
> For some greater resolution, the rdma threshold
> variable will overflow.
> 
> Signed-off-by: Bibby Hsieh <bibby.hsieh@...iatek.com>
> ---
>  drivers/gpu/drm/mediatek/mtk_disp_rdma.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_disp_rdma.c b/drivers/gpu/drm/mediatek/mtk_disp_rdma.c
> index 0df05f9..9afdcd7 100644
> --- a/drivers/gpu/drm/mediatek/mtk_disp_rdma.c
> +++ b/drivers/gpu/drm/mediatek/mtk_disp_rdma.c
> @@ -37,7 +37,7 @@
>  #define DISP_REG_RDMA_FIFO_CON			0x0040
>  #define RDMA_FIFO_UNDERFLOW_EN				BIT(31)
>  #define RDMA_FIFO_PSEUDO_SIZE(bytes)			(((bytes) / 16) << 16)
> -#define RDMA_OUTPUT_VALID_FIFO_THRESHOLD(bytes)		((bytes) / 16)
> +#define RDMA_OUTPUT_VALID_FIFO_THRESHOLD(bytes) (((bytes) / 16) & 0x3ff)

I agree with the earlier comment; clamp_val() might be more appropriate here
and would avoid unexpected results.

>  
>  /**
>   * struct mtk_disp_rdma - DISP_RDMA driver structure
> @@ -109,7 +109,7 @@ static void mtk_rdma_config(struct mtk_ddp_comp *comp, unsigned int width,
>  			    unsigned int height, unsigned int vrefresh,
>  			    unsigned int bpc)
>  {
> -	unsigned int threshold;
> +	unsigned long long threshold;
>  	unsigned int reg;
>  
>  	rdma_update_bits(comp, DISP_REG_RDMA_SIZE_CON_0, 0xfff, width);
> @@ -121,7 +121,8 @@ static void mtk_rdma_config(struct mtk_ddp_comp *comp, unsigned int width,
>  	 * output threshold to 6 microseconds with 7/6 overhead to
>  	 * account for blanking, and with a pixel depth of 4 bytes:
>  	 */
> -	threshold = width * height * vrefresh * 4 * 7 / 1000000;
> +	threshold = (unsigned long long)width * height * vrefresh *
> +		    4 * 7 / 1000000;

This is a 64 bit divide operation. It will result in a build failure
if compiled for a 32 bit kernel (eg arm:allmodconfig).

ERROR: "__aeabi_uldivmod" [drivers/gpu/drm/mediatek/mediatek-drm.ko] undefined!

Guenter