linux-kernel - Re: [PATCH 2/5] mfd: lm3533-ctrlbank: Cap BRIGHTNESS

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Message-ID: <20200629125102.GT3334@localhost>
Date:   Mon, 29 Jun 2020 14:51:02 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Johan Hovold <jhovold@...il.com>
Subject: Re: [PATCH 2/5] mfd: lm3533-ctrlbank: Cap BRIGHTNESS_MAX to 127
 since API uses u8 as carrier

On Mon, Jun 29, 2020 at 01:32:12PM +0100, Lee Jones wrote:
> Since its conception in 2012 brightness has been artificially capped
> at 127 since the variable carrying the value is u8.  We could go to
> the trouble of changing the whole API (crossing 3 different subsystems),
> but clearly this hasn't bothered anyone in the best part of a decade.
> 
> Simply, cap BRIGHTNESS_MAX to 127 instead (for now at least).

Hmm. This patch is clearly broken and would contrary to the claim be
introducing an artificial cap at half brightness. u8 can hold the max
brightness value 255 just fine.

> Fixes the following W=1 warning(s):
> 
>  drivers/mfd/lm3533-ctrlbank.c: In function ‘lm3533_ctrlbank_set_brightness’:
>  drivers/mfd/lm3533-ctrlbank.c:98:10: warning: comparison is always false due to limited range of data type [-Wtype-limits]
>  98 | if (val > LM3533_##_NAME##_MAX) | ^
>  drivers/mfd/lm3533-ctrlbank.c:125:1: note: in expansion of macro ‘lm3533_ctrlbank_set’
>  125 | lm3533_ctrlbank_set(brightness, BRIGHTNESS);
>  | ^~~~~~~~~~~~~~~~~~~

This warning is benign. The same macro is used to defined two function
where in one case the max value coincides with U8_MAX so that the sanity
check becomes redundant.

> Cc: Johan Hovold <jhovold@...il.com>
> Signed-off-by: Lee Jones <lee.jones@...aro.org>
> ---
>  drivers/mfd/lm3533-ctrlbank.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mfd/lm3533-ctrlbank.c b/drivers/mfd/lm3533-ctrlbank.c
> index 34fba06ec7057..348ce67453092 100644
> --- a/drivers/mfd/lm3533-ctrlbank.c
> +++ b/drivers/mfd/lm3533-ctrlbank.c
> @@ -17,7 +17,7 @@
>  #define LM3533_MAX_CURRENT_MAX		29800
>  #define LM3533_MAX_CURRENT_STEP		800
>  
> -#define LM3533_BRIGHTNESS_MAX		255
> +#define LM3533_BRIGHTNESS_MAX		127  /* Capped by API - could be up to 255 */
>  #define LM3533_PWM_MAX			0x3f
>  
>  #define LM3533_REG_PWM_BASE		0x14

Johan