linux-kernel - Re: [PATCH -next] drm/nouveau/clk: Use plain "/" for 32-bit division

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Message-ID: <CAMuHMdXbac9+oeVfqMpqGVfHaj9MbQ8=XeksKgnMiA8hPb=imw@mail.gmail.com>
Date:	Fri, 6 Feb 2015 08:53:04 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Alexandre Courbot <acourbot@...dia.com>
Cc:	David Airlie <airlied@...ux.ie>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-Next <linux-next@...r.kernel.org>
Subject: Re: [PATCH -next] drm/nouveau/clk: Use plain "/" for 32-bit division

Hi Alexandre,

On Fri, Feb 6, 2015 at 7:50 AM, Alexandre Courbot <acourbot@...dia.com> wrote:
> On 02/05/2015 01:02 AM, Geert Uytterhoeven wrote:
>> On 32-bit platforms using asm-generic/div64.h:
>> drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c: In function
>> 'gk20a_pllg_calc_rate':
>> drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c:147:79: warning:
>> comparison of distinct pointer types lacks a cast
>>    do_div(rate, divider);
>>
>> ^
>> drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c:147:2: warning: right
>> shift count >= width of type
>>    do_div(rate, divider);
>>    ^
>> drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c:147:238: warning: passing
>> argument 1 of '__div64_32' from incompatible pointer type
>>    do_div(rate, divider);
>>
>> ^
>> In file included from arch/parisc/include/generated/asm/div64.h:1:0,
>>                   from include/linux/kernel.h:124,
>>                   from include/linux/list.h:8,
>>                   from include/linux/preempt.h:10,
>>                   from include/linux/spinlock.h:50,
>>                   from include/linux/mmzone.h:7,
>>                   from include/linux/gfp.h:5,
>>                   from include/linux/slab.h:14,
>>                   from drivers/gpu/drm/nouveau/include/nvif/os.h:5,
>>                   from drivers/gpu/drm/nouveau/include/nvkm/core/os.h:3,
>>                   from
>> drivers/gpu/drm/nouveau/include/nvkm/core/object.h:3,
>>                   from
>> drivers/gpu/drm/nouveau/include/nvkm/core/subdev.h:3,
>>                   from
>> drivers/gpu/drm/nouveau/include/nvkm/subdev/clk.h:3,
>>                   from drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c:25:
>> include/asm-generic/div64.h:35:17: note: expected 'uint64_t *' but
>> argument is of type 'u32 *'
>>   extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
>>                   ^
>>
>> do_div() is meant for 64-bit by 32-bit division, but both the dividend
>> and divisor are 32-bit here. Hence use plain "/" instead.
>>
>> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>> ---
>> Compile-tested only.
>>
>> parisc/allmodconfig:
>> http://kisskb.ellerman.id.au/kisskb/buildresult/12358386/
>> ---
>>   drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c
>> b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c
>> index 65c532742b08d1c6..022595876ea4dc85 100644
>> --- a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c
>> +++ b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c
>> @@ -144,9 +144,8 @@ gk20a_pllg_calc_rate(struct gk20a_clk_priv *priv)
>>
>>         rate = priv->parent_rate * priv->n;
>>         divider = priv->m * pl_to_div[priv->pl];
>> -       do_div(rate, divider);
>>
>> -       return rate / 2;
>> +       return rate / divider / 2;
>
>
> I agree there is a problem here, but considering the theoretical values that
> rate can take, could we rather fix this by making rate a u64? With the
> current maximum values of priv->parent_rate and priv->n, rate might approach
> dangerously to the u32 limit and I believe casting it to u64 would be safer.

Yes, that's better.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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