linux-kernel - Re: [PATCH 1/9] backlight: atmel-pwm-bl: fix reported brightness

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Date:	Wed, 23 Oct 2013 10:51:28 +0200
From:	Johan Hovold <jhovold@...il.com>
To:	Jingoo Han <jg1.han@...sung.com>
Cc:	'Johan Hovold' <jhovold@...il.com>,
	'Richard Purdie' <rpurdie@...ys.net>,
	'Nicolas Ferre' <nicolas.ferre@...el.com>,
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/9] backlight: atmel-pwm-bl: fix reported brightness

On Wed, Oct 23, 2013 at 10:20:59AM +0900, Jingoo Han wrote:
> On Wednesday, October 23, 2013 2:27 AM, Johan Hovold wrote:
> > 
> > The driver supports 16-bit brightness values, but the value returned
> > from get_brightness was truncated to eight bits.
> > 
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Johan Hovold <jhovold@...il.com>
> > ---
> >  drivers/video/backlight/atmel-pwm-bl.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/video/backlight/atmel-pwm-bl.c b/drivers/video/backlight/atmel-pwm-bl.c
> > index 66885fb..8aac273 100644
> > --- a/drivers/video/backlight/atmel-pwm-bl.c
> > +++ b/drivers/video/backlight/atmel-pwm-bl.c
> > @@ -70,7 +70,7 @@ static int atmel_pwm_bl_set_intensity(struct backlight_device *bd)
> >  static int atmel_pwm_bl_get_intensity(struct backlight_device *bd)
> >  {
> >  	struct atmel_pwm_bl *pwmbl = bl_get_data(bd);
> > -	u8 intensity;
> > +	u32 intensity;
> > 
> >  	if (pwmbl->pdata->pwm_active_low) {
> >  		intensity = pwm_channel_readl(&pwmbl->pwmc, PWM_CDTY) -
> > @@ -80,7 +80,7 @@ static int atmel_pwm_bl_get_intensity(struct backlight_device *bd)
> >  			pwm_channel_readl(&pwmbl->pwmc, PWM_CDTY);
> >  	}
> > 
> > -	return intensity;
> > +	return (u16)intensity;
> 
> However, atmel_pwm_bl_get_intensity() should return 'int',
> instead of 'u16'.

Yes, but the cast to int is implicit. Perhaps

	return (intensity & 0xffff);

(or just a comment) would make it more clear why the cast is there.

> Also, pwm_channel_readl() returns 'u32'.

Yes, (and only the 16 least-significant bits are used). That and the
fact that the platform-data limits are currently unsigned long (I was
considering fixing this later) was why I preferred keeping all register
value manipulation in u32 and do a single cast at the end.

> Then, how about the following?
> 
> --- a/drivers/video/backlight/atmel-pwm-bl.c
> +++ b/drivers/video/backlight/atmel-pwm-bl.c
> @@ -70,17 +70,17 @@ static int atmel_pwm_bl_set_intensity(struct backlight_device *bd)
>  static int atmel_pwm_bl_get_intensity(struct backlight_device *bd)
>  {
> 	struct atmel_pwm_bl *pwmbl = bl_get_data(bd);
> -	u8 intensity;
> +	u16 intensity;
> 
> 	if (pwmbl->pdata->pwm_active_low) {
> -		intensity = pwm_channel_readl(&pwmbl->pwmc, PWM_CDTY) -
> +		intensity = (u16) pwm_channel_readl(&pwmbl->pwmc, PWM_CDTY) -
> 			pwmbl->pdata->pwm_duty_min;

This would actually introduce a new conversion warning (unless you add
parentheses as well) as pwm_duty_min is unsigned long. Same below.

> 	} else {
> -		intensity = pwmbl->pdata->pwm_duty_max -
> +		intensity = (u16) pwmbl->pdata->pwm_duty_max -
> 			pwm_channel_readl(&pwmbl->pwmc, PWM_CDTY);
> 	}
> 
> -	return intensity;
> +	return (int)intensity;
>  }

However, if you feel strongly about this, I'll respin the series (a later
patch would likely no longer apply), use u16 and add casts to the two
assignments.

Thanks,
Johan
 
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