Nouveau - Sep 2017 - [PATCH] [RESEND] drm/nouveau/clk: fix gcc-7 -Wint-in-bool-context warning

gcc thinks that interpreting a multiplication result as a bool
is confusing:

drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c: In function 'read_pll':
drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c:133:8: error: '*' in
boolean context, suggest '&&' instead
[-Werror=int-in-bool-context]

In this instance, I think using multiplication is more intuitive
than '&&', so I'm adding a comparison to zero instead to
shut up
the warning. To further improve readability, I also make the
error case indented and leave the normal case as the final 'return'
statement.

Fixes: 7632b30e4b8b ("drm/nouveau/clk: namespace + nvidia gpu names (no
binary change)")
Signed-off-by: Arnd Bergmann <arnd at arndb.de>
---
Originally submitted on July 14, but no reply. This is the same
patch again. The warning is currently disabled in mainline, but
I think we can turn it back on in the future, and this change here
seems harmless.
---
 drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c
b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c
index 96e0941c8edd..04b4f4ccf186 100644
--- a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c
+++ b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c
@@ -130,10 +130,10 @@ read_pll(struct gt215_clk *clk, int idx, u32 pll)
 		sclk = read_clk(clk, 0x10 + idx, false);
 	}
 
-	if (M * P)
-		return sclk * N / (M * P);
+	if (M * P == 0)
+		return 0;
 
-	return 0;
+	return sclk * N / (M * P);
 }
 
 static int
-- 
2.9.0

On Wed, Sep 6, 2017 at 3:56 PM, Arnd Bergmann <arnd at arndb.de>
wrote:
> gcc thinks that interpreting a multiplication result as a bool
> is confusing:
>
> drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c: In function
'read_pll':
> drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c:133:8: error: '*'
in boolean context, suggest '&&' instead
[-Werror=int-in-bool-context]
>
> In this instance, I think using multiplication is more intuitive
> than '&&', so I'm adding a comparison to zero instead
to shut up
> the warning. To further improve readability, I also make the
> error case indented and leave the normal case as the final 'return'
> statement.
>
I think to make perfectly clear why this check is done, we simply
should precompute the denominator and do something like this:

int denominator = M * P
if (denominator == 0)
   return 0;
return sclk * N / denominator;

but with a better name for "denominator". I even imagine, that there
are some macros in the kernel for dealing with something like this
already, so that we could do instead:

return AWESOME_DIV_MACRO(sclk * N, M * P)
> Fixes: 7632b30e4b8b ("drm/nouveau/clk: namespace + nvidia gpu names
(no binary change)")
> Signed-off-by: Arnd Bergmann <arnd at arndb.de>
> ---
> Originally submitted on July 14, but no reply. This is the same
> patch again. The warning is currently disabled in mainline, but
> I think we can turn it back on in the future, and this change here
> seems harmless.
> ---
>  drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c
b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c
> index 96e0941c8edd..04b4f4ccf186 100644
> --- a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c
> +++ b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c
> @@ -130,10 +130,10 @@ read_pll(struct gt215_clk *clk, int idx, u32 pll)
>                 sclk = read_clk(clk, 0x10 + idx, false);
>         }
>
> -       if (M * P)
> -               return sclk * N / (M * P);
> +       if (M * P == 0)
> +               return 0;
>
> -       return 0;
> +       return sclk * N / (M * P);
>  }
>
>  static int
> --
> 2.9.0
>
> _______________________________________________
> Nouveau mailing list
> Nouveau at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/nouveau

On Wed, Sep 6, 2017 at 4:20 PM, Karol Herbst <karolherbst at gmail.com>
wrote:
>> In this instance, I think using multiplication is more intuitive
>> than '&&', so I'm adding a comparison to zero
instead to shut up
>> the warning. To further improve readability, I also make the
>> error case indented and leave the normal case as the final
'return'
>> statement.
>>
>
> I think to make perfectly clear why this check is done, we simply
> should precompute the denominator and do something like this:
>
> int denominator = M * P
> if (denominator == 0)
>    return 0;
> return sclk * N / denominator;
>
> but with a better name for "denominator".
I don't know what M and P actually are in this function, so I couldn't
come up with a much better name either, how about simply 'divisor'?
> I even imagine, that there
> are some macros in the kernel for dealing with something like this
> already, so that we could do instead:
>
> return AWESOME_DIV_MACRO(sclk * N, M * P)
I don't think that exists, the behavior for divide-by-zero really
depends a lot on the context, and returning zero is probably
often not a good solution.

      Arnd