Re: [PATCH] nouveau/nvkm/subdev/clk/gk20a.c: fix wrong do_div() usage

From: Thierry Reding <thierry.reding@gmail.com>
To: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Ben Skeggs <bskeggs@redhat.com>, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH] nouveau/nvkm/subdev/clk/gk20a.c: fix wrong do_div() usage
Date: Fri, 4 Dec 2015 18:38:01 +0100	[thread overview]
Message-ID: <20151204173801.GA30117@ulmo.nvidia.com> (raw)
In-Reply-To: <alpine.LFD.2.20.1511031653580.630@knanqh.ubzr>

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On Tue, Nov 03, 2015 at 05:01:46PM -0500, Nicolas Pitre wrote:
> do_div() must only be used with a u64 dividend.
> 
> Signed-off-by: Nicolas Pitre <nico@linaro.org>
> 
> diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c
> index 254094ab7f..5da2aa8cc3 100644
> --- a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c
> +++ b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c
> @@ -141,9 +141,8 @@ gk20a_pllg_calc_rate(struct gk20a_clk *clk)
>  
>  	rate = clk->parent_rate * clk->n;
>  	divider = clk->m * pl_to_div[clk->pl];
> -	do_div(rate, divider);
>  
> -	return rate / 2;
> +	return rate / divider / 2;
>  }
>  
>  static int

This causes build breakage on 32-bit ARM. I'm also confused by the
commit message because the code that I'm looking at has u64 rate and u32
divider, which matches the types given in include/asm-generic/div64.h:

 * The semantics of do_div() are:
 *
 * uint32_t do_div(uint64_t *n, uint32_t base)
 * {
 * 	uint32_t remainder = *n % base;
 * 	*n = *n / base;
 * 	return remainder;
 * }

Thierry

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