[PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Nathan Chancellor]

When building with -Wsometimes-uninitialized, Clang warns:

drivers/pwm/pwm-img.c:126:13: error: variable 'timebase' is used
uninitialized whenever 'if' condition is false
[-Werror,-Wsometimes-uninitialized]

The final else if functions as an else; make that explicit so that Clang
understands that timebase cannot be used uninitialized.

Link: https://github.com/ClangBuiltLinux/linux/issues/400
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
---
 drivers/pwm/pwm-img.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/pwm/pwm-img.c b/drivers/pwm/pwm-img.c
index 815f5333bb8f..1cc5fbe1e1d3 100644
--- a/drivers/pwm/pwm-img.c
+++ b/drivers/pwm/pwm-img.c
@@ -123,7 +123,7 @@ static int img_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	} else if (mul <= max_timebase * 512) {
 		div = PWM_CTRL_CFG_SUB_DIV0_DIV1;
 		timebase = DIV_ROUND_UP(mul, 512);
-	} else if (mul > max_timebase * 512) {
+	} else {
 		dev_err(chip->dev,
 			"failed to configure timebase steps/divider value\n");
 		return -EINVAL;
-- 
2.21.0

Re: [PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Nick Desaulniers]

On Thu, Mar 7, 2019 at 2:36 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> When building with -Wsometimes-uninitialized, Clang warns:
>
> drivers/pwm/pwm-img.c:126:13: error: variable 'timebase' is used
> uninitialized whenever 'if' condition is false
> [-Werror,-Wsometimes-uninitialized]
>
> The final else if functions as an else; make that explicit so that Clang
> understands that timebase cannot be used uninitialized.

A welcome simplification! Thank you.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
> Link: https://github.com/ClangBuiltLinux/linux/issues/400
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
>  drivers/pwm/pwm-img.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/pwm/pwm-img.c b/drivers/pwm/pwm-img.c
> index 815f5333bb8f..1cc5fbe1e1d3 100644
> --- a/drivers/pwm/pwm-img.c
> +++ b/drivers/pwm/pwm-img.c
> @@ -123,7 +123,7 @@ static int img_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>         } else if (mul <= max_timebase * 512) {
>                 div = PWM_CTRL_CFG_SUB_DIV0_DIV1;
>                 timebase = DIV_ROUND_UP(mul, 512);
> -       } else if (mul > max_timebase * 512) {
> +       } else {
>                 dev_err(chip->dev,
>                         "failed to configure timebase steps/divider value\n");
>                 return -EINVAL;
> --
> 2.21.0
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Uwe Kleine-König]

On Thu, Mar 07, 2019 at 03:36:28PM -0700, Nathan Chancellor wrote:
> When building with -Wsometimes-uninitialized, Clang warns:
> 
> drivers/pwm/pwm-img.c:126:13: error: variable 'timebase' is used
> uninitialized whenever 'if' condition is false
> [-Werror,-Wsometimes-uninitialized]
> 
> The final else if functions as an else; make that explicit so that Clang
> understands that timebase cannot be used uninitialized.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/400
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
>  drivers/pwm/pwm-img.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pwm/pwm-img.c b/drivers/pwm/pwm-img.c
> index 815f5333bb8f..1cc5fbe1e1d3 100644
> --- a/drivers/pwm/pwm-img.c
> +++ b/drivers/pwm/pwm-img.c
> @@ -123,7 +123,7 @@ static int img_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>  	} else if (mul <= max_timebase * 512) {
>  		div = PWM_CTRL_CFG_SUB_DIV0_DIV1;
>  		timebase = DIV_ROUND_UP(mul, 512);
> -	} else if (mul > max_timebase * 512) {
> +	} else {
>  		dev_err(chip->dev,
>  			"failed to configure timebase steps/divider value\n");
>  		return -EINVAL;

This can even be simplified further.

From the probe function we have:

	pwm_chip->max_period_ns = NSEC_PER_SEC * 512 * max_timebase / input_clk_hz

Then in img_pwm_config there is:

	mul = ⎡ input_clk_hz / ⎡ NSEC_PER_SEC / period_ns⎤⎤

(Not sure this term is the best we can come up with. The rounding
strategy looks strange because the first DIV_ROUND_UP makes mul smaller
while the second makes it bigger. This is similar to

	⎡ input_clk_hz * period_ns / NSEC_PER_SEC ⎤

which is probably more exact and cheaper to calculate.)

If we now had

	mul > max_timebase * 512

this results in (apart from rounding errors):

	input_clk_hz * period_ns / NSEC_PER_SEC > max_timebase * 512
 <=>    period_ns > max_timebase * 512 * NSEC_PER_SEC / input_clk_hz
 <=>    period_ns > pwm_chip->max_period_ns

This however is already ruled out by the first check in
img_pwm_config().

So if the rounding would be fixed, the else is a dead branch and could
be dropped completely.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

Re: [PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Nick Desaulniers]

On Fri, Mar 8, 2019 at 12:53 AM Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
>
> On Thu, Mar 07, 2019 at 03:36:28PM -0700, Nathan Chancellor wrote:
> > When building with -Wsometimes-uninitialized, Clang warns:
> >
> > drivers/pwm/pwm-img.c:126:13: error: variable 'timebase' is used
> > uninitialized whenever 'if' condition is false
> > [-Werror,-Wsometimes-uninitialized]
> >
> > The final else if functions as an else; make that explicit so that Clang
> > understands that timebase cannot be used uninitialized.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/400
> > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > ---
> >  drivers/pwm/pwm-img.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/pwm/pwm-img.c b/drivers/pwm/pwm-img.c
> > index 815f5333bb8f..1cc5fbe1e1d3 100644
> > --- a/drivers/pwm/pwm-img.c
> > +++ b/drivers/pwm/pwm-img.c
> > @@ -123,7 +123,7 @@ static int img_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> >       } else if (mul <= max_timebase * 512) {
> >               div = PWM_CTRL_CFG_SUB_DIV0_DIV1;
> >               timebase = DIV_ROUND_UP(mul, 512);
> > -     } else if (mul > max_timebase * 512) {
> > +     } else {
> >               dev_err(chip->dev,
> >                       "failed to configure timebase steps/divider value\n");
> >               return -EINVAL;
>
> This can even be simplified further.
>
> From the probe function we have:
>
>         pwm_chip->max_period_ns = NSEC_PER_SEC * 512 * max_timebase / input_clk_hz

I had trouble verifying `input_clk_hz` in the above.  The divisor is
`clk_get_rate(pwm->pwm_clk)`, but is it guaranteed to always be
`input_clk_hz`? If so, where?

>
> Then in img_pwm_config there is:
>
>         mul = ⎡ input_clk_hz / ⎡ NSEC_PER_SEC / period_ns⎤⎤
>
> (Not sure this term is the best we can come up with. The rounding
> strategy looks strange because the first DIV_ROUND_UP makes mul smaller
> while the second makes it bigger. This is similar to
>
>         ⎡ input_clk_hz * period_ns / NSEC_PER_SEC ⎤
>
> which is probably more exact and cheaper to calculate.)
>
> If we now had
>
>         mul > max_timebase * 512
>
> this results in (apart from rounding errors):
>
>         input_clk_hz * period_ns / NSEC_PER_SEC > max_timebase * 512
>  <=>    period_ns > max_timebase * 512 * NSEC_PER_SEC / input_clk_hz
>  <=>    period_ns > pwm_chip->max_period_ns
>
> This however is already ruled out by the first check in
> img_pwm_config().
>
> So if the rounding would be fixed, the else is a dead branch and could
> be dropped completely.

-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Uwe Kleine-König]

On Fri, Mar 08, 2019 at 10:38:11AM -0800, Nick Desaulniers wrote:
> On Fri, Mar 8, 2019 at 12:53 AM Uwe Kleine-König
> <u.kleine-koenig@pengutronix.de> wrote:
> >
> > On Thu, Mar 07, 2019 at 03:36:28PM -0700, Nathan Chancellor wrote:
> > > When building with -Wsometimes-uninitialized, Clang warns:
> > >
> > > drivers/pwm/pwm-img.c:126:13: error: variable 'timebase' is used
> > > uninitialized whenever 'if' condition is false
> > > [-Werror,-Wsometimes-uninitialized]
> > >
> > > The final else if functions as an else; make that explicit so that Clang
> > > understands that timebase cannot be used uninitialized.
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/400
> > > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > > ---
> > >  drivers/pwm/pwm-img.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/pwm/pwm-img.c b/drivers/pwm/pwm-img.c
> > > index 815f5333bb8f..1cc5fbe1e1d3 100644
> > > --- a/drivers/pwm/pwm-img.c
> > > +++ b/drivers/pwm/pwm-img.c
> > > @@ -123,7 +123,7 @@ static int img_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > >       } else if (mul <= max_timebase * 512) {
> > >               div = PWM_CTRL_CFG_SUB_DIV0_DIV1;
> > >               timebase = DIV_ROUND_UP(mul, 512);
> > > -     } else if (mul > max_timebase * 512) {
> > > +     } else {
> > >               dev_err(chip->dev,
> > >                       "failed to configure timebase steps/divider value\n");
> > >               return -EINVAL;
> >
> > This can even be simplified further.
> >
> > From the probe function we have:
> >
> >         pwm_chip->max_period_ns = NSEC_PER_SEC * 512 * max_timebase / input_clk_hz
> 
> I had trouble verifying `input_clk_hz` in the above.  The divisor is
> `clk_get_rate(pwm->pwm_clk)`, but is it guaranteed to always be
> `input_clk_hz`? If so, where?

In the probe function it's called "clk_rate". We have:

	clk_rate = clk_get_rate(pwm->pwm_clk);
	...
	val = (u64)NSEC_PER_SEC * 512 * pwm->data->max_timebase;
	do_div(val, clk_rate);
	pwm->max_period_ns = val;

and in img_pwm_config we have:

	input_clk_hz = clk_get_rate(pwm_chip->pwm_clk);

I used the name used in img_pwm_config with the intention that my
reasoning is easier to understand, but obviously I failed.

This by the way highlights another patch opportunity: Unify the names
that the same thing gets the same name in the different functions. This
doesn't only affect "clk_rate" vs "input_clk_hz", but also "pwm_chip"
vs. "pwm".

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

Re: [PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Nick Desaulniers]

On Fri, Mar 8, 2019 at 1:18 PM Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
>
> On Fri, Mar 08, 2019 at 10:38:11AM -0800, Nick Desaulniers wrote:
> > On Fri, Mar 8, 2019 at 12:53 AM Uwe Kleine-König
> > <u.kleine-koenig@pengutronix.de> wrote:
> > >
> > > On Thu, Mar 07, 2019 at 03:36:28PM -0700, Nathan Chancellor wrote:
> > > > When building with -Wsometimes-uninitialized, Clang warns:
> > > >
> > > > drivers/pwm/pwm-img.c:126:13: error: variable 'timebase' is used
> > > > uninitialized whenever 'if' condition is false
> > > > [-Werror,-Wsometimes-uninitialized]
> > > >
> > > > The final else if functions as an else; make that explicit so that Clang
> > > > understands that timebase cannot be used uninitialized.
> > > >
> > > > Link: https://github.com/ClangBuiltLinux/linux/issues/400
> > > > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > > > ---
> > > >  drivers/pwm/pwm-img.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/pwm/pwm-img.c b/drivers/pwm/pwm-img.c
> > > > index 815f5333bb8f..1cc5fbe1e1d3 100644
> > > > --- a/drivers/pwm/pwm-img.c
> > > > +++ b/drivers/pwm/pwm-img.c
> > > > @@ -123,7 +123,7 @@ static int img_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > > >       } else if (mul <= max_timebase * 512) {
> > > >               div = PWM_CTRL_CFG_SUB_DIV0_DIV1;
> > > >               timebase = DIV_ROUND_UP(mul, 512);
> > > > -     } else if (mul > max_timebase * 512) {
> > > > +     } else {
> > > >               dev_err(chip->dev,
> > > >                       "failed to configure timebase steps/divider value\n");
> > > >               return -EINVAL;
> > >
> > > This can even be simplified further.
> > >
> > > From the probe function we have:
> > >
> > >         pwm_chip->max_period_ns = NSEC_PER_SEC * 512 * max_timebase / input_clk_hz
> >
> > I had trouble verifying `input_clk_hz` in the above.  The divisor is
> > `clk_get_rate(pwm->pwm_clk)`, but is it guaranteed to always be
> > `input_clk_hz`? If so, where?
>
> In the probe function it's called "clk_rate". We have:
>
>         clk_rate = clk_get_rate(pwm->pwm_clk);
>         ...
>         val = (u64)NSEC_PER_SEC * 512 * pwm->data->max_timebase;
>         do_div(val, clk_rate);
>         pwm->max_period_ns = val;
>
> and in img_pwm_config we have:
>
>         input_clk_hz = clk_get_rate(pwm_chip->pwm_clk);
>
> I used the name used in img_pwm_config with the intention that my
> reasoning is easier to understand, but obviously I failed.
>
> This by the way highlights another patch opportunity: Unify the names
> that the same thing gets the same name in the different functions. This
> doesn't only affect "clk_rate" vs "input_clk_hz", but also "pwm_chip"
> vs. "pwm".

Sure, those seem like nice little cleanups that will help with
maintainability.  Would you mind sending a patch for those, and I'll
review? Otherwise Nathan's patch exists and is ready to go.
-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Uwe Kleine-König]

On Fri, Mar 08, 2019 at 01:23:41PM -0800, Nick Desaulniers wrote:
> On Fri, Mar 8, 2019 at 1:18 PM Uwe Kleine-König
> <u.kleine-koenig@pengutronix.de> wrote:
> >
> > On Fri, Mar 08, 2019 at 10:38:11AM -0800, Nick Desaulniers wrote:
> > > On Fri, Mar 8, 2019 at 12:53 AM Uwe Kleine-König
> > > <u.kleine-koenig@pengutronix.de> wrote:
> > > >
> > > > On Thu, Mar 07, 2019 at 03:36:28PM -0700, Nathan Chancellor wrote:
> > > > > When building with -Wsometimes-uninitialized, Clang warns:
> > > > >
> > > > > drivers/pwm/pwm-img.c:126:13: error: variable 'timebase' is used
> > > > > uninitialized whenever 'if' condition is false
> > > > > [-Werror,-Wsometimes-uninitialized]
> > > > >
> > > > > The final else if functions as an else; make that explicit so that Clang
> > > > > understands that timebase cannot be used uninitialized.
> > > > >
> > > > > Link: https://github.com/ClangBuiltLinux/linux/issues/400
> > > > > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > > > > ---
> > > > >  drivers/pwm/pwm-img.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/pwm/pwm-img.c b/drivers/pwm/pwm-img.c
> > > > > index 815f5333bb8f..1cc5fbe1e1d3 100644
> > > > > --- a/drivers/pwm/pwm-img.c
> > > > > +++ b/drivers/pwm/pwm-img.c
> > > > > @@ -123,7 +123,7 @@ static int img_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > > > >       } else if (mul <= max_timebase * 512) {
> > > > >               div = PWM_CTRL_CFG_SUB_DIV0_DIV1;
> > > > >               timebase = DIV_ROUND_UP(mul, 512);
> > > > > -     } else if (mul > max_timebase * 512) {
> > > > > +     } else {
> > > > >               dev_err(chip->dev,
> > > > >                       "failed to configure timebase steps/divider value\n");
> > > > >               return -EINVAL;
> > > >
> > > > This can even be simplified further.
> > > >
> > > > From the probe function we have:
> > > >
> > > >         pwm_chip->max_period_ns = NSEC_PER_SEC * 512 * max_timebase / input_clk_hz
> > >
> > > I had trouble verifying `input_clk_hz` in the above.  The divisor is
> > > `clk_get_rate(pwm->pwm_clk)`, but is it guaranteed to always be
> > > `input_clk_hz`? If so, where?
> >
> > In the probe function it's called "clk_rate". We have:
> >
> >         clk_rate = clk_get_rate(pwm->pwm_clk);
> >         ...
> >         val = (u64)NSEC_PER_SEC * 512 * pwm->data->max_timebase;
> >         do_div(val, clk_rate);
> >         pwm->max_period_ns = val;
> >
> > and in img_pwm_config we have:
> >
> >         input_clk_hz = clk_get_rate(pwm_chip->pwm_clk);
> >
> > I used the name used in img_pwm_config with the intention that my
> > reasoning is easier to understand, but obviously I failed.
> >
> > This by the way highlights another patch opportunity: Unify the names
> > that the same thing gets the same name in the different functions. This
> > doesn't only affect "clk_rate" vs "input_clk_hz", but also "pwm_chip"
> > vs. "pwm".
> 
> Sure, those seem like nice little cleanups that will help with
> maintainability.  Would you mind sending a patch for those, and I'll
> review? Otherwise Nathan's patch exists and is ready to go.

I don't mind. But I would prefer to get some input from Thierry about
how to round first. I also wonder if there is a publically available
manual for this piece of hardware.

Best regards
Uwe



	

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

Re: [PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Uwe Kleine-König]

Hello Nathan,

the copy of my mail sent to clang-built-linux@googlegroups.com was not
accepted. (So I dropped this address from the recipients for this
reply.) Probably this list isn't open for non-members to post to. This
is a bit annoying, so I ask you to either fix that or don't add this
address to the recipients of your posts where non-members might answer.

Thanks
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

Re: [PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Nick Desaulniers]

On Fri, Mar 8, 2019 at 12:58 AM Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
>
> Hello Nathan,
>
> the copy of my mail sent to clang-built-linux@googlegroups.com was not
> accepted. (So I dropped this address from the recipients for this
> reply.) Probably this list isn't open for non-members to post to. This
> is a bit annoying, so I ask you to either fix that or don't add this
> address to the recipients of your posts where non-members might answer.

Ah, sorry, I did not realize that it would annoy people like this when
I set it up.  We've email vger post master to get us set up with a
list.  No response yet.
-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] pwm: img: Turn final 'else if' into 'else' in img_pwm_config

[Thierry Reding]

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On Thu, Mar 07, 2019 at 03:36:28PM -0700, Nathan Chancellor wrote:
> When building with -Wsometimes-uninitialized, Clang warns:
> 
> drivers/pwm/pwm-img.c:126:13: error: variable 'timebase' is used
> uninitialized whenever 'if' condition is false
> [-Werror,-Wsometimes-uninitialized]
> 
> The final else if functions as an else; make that explicit so that Clang
> understands that timebase cannot be used uninitialized.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/400
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
>  drivers/pwm/pwm-img.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Applied, thanks!

Thierry

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