Re: [PATCH 2/2] clk/vt8500: Fix compilation warnings

From: Arnd Bergmann
Date: Tue May 31 2016 - 06:51:54 EST

On Tuesday, May 24, 2016 11:07:53 PM CEST Roman Volkov wrote:
> From: Roman Volkov <rvolkov@xxxxxxxxx>
>
> GCC 5.3.0 still throws the following warnings for functions
> wm8750_find_pll_bits() and wm8850_find_pll_bits():
>
> warning: 'best_div2' may be used uninitialized in this function
> warning: 'best_div1' may be used uninitialized in this function
> warning: 'best_mul' may be used uninitialized in this function
>
> These warnings are false positives, the variables are controlled
> by checking the value of the variable 'best_err' which is -1 by
> default. It is safe to initialize all these variables to zero.
>
> Fixes: 090341b0a95d ("clk: vt8500: fix sign of possible PLL values")
> Signed-off-by: Roman Volkov <rvolkov@xxxxxxxxx>

With gcc-5.3 and linux-4.7-rc1, they should be gone now (no longer
warning about this with gcov enabled), but I still get them with
gcc-4.9.

> diff --git a/drivers/clk/clk-vt8500.c b/drivers/clk/clk-vt8500.c
> index 77650f19a9b6..7c970d7c0a6a 100644
> --- a/drivers/clk/clk-vt8500.c
> +++ b/drivers/clk/clk-vt8500.c
> @@ -461,7 +461,7 @@ static int wm8750_find_pll_bits(unsigned long rate, unsigned long parent_rate,
> {
> u32 mul;
> int div1, div2;
> - u32 best_mul, best_div1, best_div2;
> + u32 best_mul = 0, best_div1 = 0, best_div2 = 0;
> unsigned long tclk, rate_err, best_err;
>
> best_err = (unsigned long)-1;
> @@ -513,7 +513,7 @@ static int wm8850_find_pll_bits(unsigned long rate, unsigned long parent_rate,
> {
> u32 mul;
> int div1, div2;
> - u32 best_mul, best_div1, best_div2;
> + u32 best_mul = 0, best_div1 = 0, best_div2 = 0;
> unsigned long tclk, rate_err, best_err;
>
> best_err = (unsigned long)-1;
>

I see you only patch two instances but not the third one. I think
we should do it consistently at least.

Coincidentally, I've just done another patch for this myself, since
it's the last gcc-4.9 warning we get in linux-4.7.

My version below

commit a38daeb34a2dc9d39ee1f153244cfcd83e865e0d
Author: Arnd Bergmann <arnd@xxxxxxxx>
Date: Tue May 31 10:41:26 2016 +0200

clk: vt8500: fix gcc-4.9 warnings

This fixes some false positive warnings we get with older compiler
versions:

clk-vt8500.c: In function âwm8650_find_pll_bitsâ:
clk-vt8500.c:430:12: âbest_div2â may be used uninitialized in this function
clk-vt8500.c:429:12: âbest_div1â may be used uninitialized in this function
clk-vt8500.c:428:14: âbest_mulâ may be used uninitialized in this function
clk-vt8500.c: In function âwm8750_find_pll_bitsâ:
clk-vt8500.c:509:12: âbest_div2â may be used uninitialized in this function
clk-vt8500.c:508:12: âbest_div1â may be used uninitialized in this function
clk-vt8500.c:507:14: âbest_mulâ may be used uninitialized in this function
clk-vt8500.c: In function âwm8850_find_pll_bitsâ:
clk-vt8500.c:560:12: âbest_div2â may be used uninitialized in this function
clk-vt8500.c:559:12: âbest_div1â may be used uninitialized in this function
clk-vt8500.c:558:14: âbest_mulâ may be used uninitialized in this function

As the local variables are only use for temporaries, we can just
as well assign the final values directly, which also makes the
code slightly shorter.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

diff --git a/drivers/clk/clk-vt8500.c b/drivers/clk/clk-vt8500.c
index b0f76a84f1e9..d5a3453970d0 100644
--- a/drivers/clk/clk-vt8500.c
+++ b/drivers/clk/clk-vt8500.c
@@ -388,7 +388,6 @@ static int wm8650_find_pll_bits(unsigned long rate, unsigned long parent_rate,
{
u32 mul, div1;
int div2;
- u32 best_mul, best_div1, best_div2;
unsigned long tclk, rate_err, best_err;

best_err = (unsigned long)-1;
@@ -411,9 +410,9 @@ static int wm8650_find_pll_bits(unsigned long rate, unsigned long parent_rate,

if (rate_err < best_err) {
best_err = rate_err;
- best_mul = mul;
- best_div1 = div1;
- best_div2 = div2;
+ *multiplier = mul;
+ *divisor1 = div1;
+ *divisor2 = div2;
}
}

@@ -425,10 +424,6 @@ static int wm8650_find_pll_bits(unsigned long rate, unsigned long parent_rate,
/* if we got here, it wasn't an exact match */
pr_warn("%s: requested rate %lu, found rate %lu\n", __func__, rate,
rate - best_err);
- *multiplier = best_mul;
- *divisor1 = best_div1;
- *divisor2 = best_div2;
-
return 0;
}

@@ -464,7 +459,6 @@ static int wm8750_find_pll_bits(unsigned long rate, unsigned long parent_rate,
{
u32 mul;
int div1, div2;
- u32 best_mul, best_div1, best_div2;
unsigned long tclk, rate_err, best_err;

best_err = (unsigned long)-1;
@@ -488,9 +482,9 @@ static int wm8750_find_pll_bits(unsigned long rate, unsigned long parent_rate,

if (rate_err < best_err) {
best_err = rate_err;
- best_mul = mul;
- best_div1 = div1;
- best_div2 = div2;
+ *multiplier = mul;
+ *divisor1 = div1;
+ *divisor2 = div2;
}
}

@@ -503,10 +497,7 @@ static int wm8750_find_pll_bits(unsigned long rate, unsigned long parent_rate,
pr_warn("%s: requested rate %lu, found rate %lu\n", __func__, rate,
rate - best_err);

- *filter = wm8750_get_filter(parent_rate, best_div1);
- *multiplier = best_mul;
- *divisor1 = best_div1;
- *divisor2 = best_div2;
+ *filter = wm8750_get_filter(parent_rate, *divisor1);

return 0;
}
@@ -516,7 +507,6 @@ static int wm8850_find_pll_bits(unsigned long rate, unsigned long parent_rate,
{
u32 mul;
int div1, div2;
- u32 best_mul, best_div1, best_div2;
unsigned long tclk, rate_err, best_err;

best_err = (unsigned long)-1;
@@ -540,9 +530,9 @@ static int wm8850_find_pll_bits(unsigned long rate, unsigned long parent_rate,

if (rate_err < best_err) {
best_err = rate_err;
- best_mul = mul;
- best_div1 = div1;
- best_div2 = div2;
+ *multiplier = mul;
+ *divisor1 = div1;
+ *divisor2 = div2;
}
}

@@ -555,10 +545,6 @@ static int wm8850_find_pll_bits(unsigned long rate, unsigned long parent_rate,
pr_warn("%s: requested rate %lu, found rate %lu\n", __func__, rate,
rate - best_err);

- *multiplier = best_mul;
- *divisor1 = best_div1;
- *divisor2 = best_div2;
-
return 0;
}