Re: [PATCH 2/5] mfd: lm3533-ctrlbank: Cap BRIGHTNESS_MAX to 127 since API uses u8 as carrier

Lee Jones
2020-06-29T19:24:00Z

On Mon, 29 Jun 2020, Johan Hovold wrote:

> 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.

Yes, of course it can. Senior moment on my account.

> > 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.

A benign warning, as most W=1 warnings are, is still a warning.

So how do you propose we fix it?

Is 255 a valid and used brightness level?

If so, how do you feel about:

/* Avoid 'always false' check '(u8) > 255' */
if (LM3533_##_NAME##_MAX != 0xff && val > LM3533_##_NAME##_MAX)
return -EINVAL;

> > Cc: Johan Hovold <jhovold@gmail.com>
> > Signed-off-by: Lee Jones <lee.jones@linaro.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

--
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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