linux-kernel - Re: [PATCH 3/3] leds: add sgm3140 driver

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Date:   Wed, 11 Mar 2020 22:09:23 +0100
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Dan Murphy <dmurphy@...com>, Luca Weiss <luca@...tu.xyz>,
        linux-leds@...r.kernel.org
Cc:     Heiko Stuebner <heiko@...ech.de>, Icenowy Zheng <icenowy@...c.io>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Ripard <mripard@...nel.org>,
        Pavel Machek <pavel@....cz>, Rob Herring <robh+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, ~postmarketos/upstreaming@...ts.sr.ht
Subject: Re: [PATCH 3/3] leds: add sgm3140 driver

Dan,

On 3/11/20 2:02 PM, Dan Murphy wrote:
> Luca
> 
> On 3/9/20 3:35 PM, Luca Weiss wrote:
>> Add a driver for the SGMICRO SGM3140 Buck/Boost Charge Pump LED driver.
>>
>> This device is controlled by two GPIO pins, one for enabling and the
>> second one for switching between torch and flash mode.
> 
> How does one enable torch and one enable flash?
> 
> Is the flash-gpio control this or does the enable-gpio enable the flash?
> 
> The DT binding did not indicate what the GPIOs are really going to control.
> 
>>
>> Signed-off-by: Luca Weiss <luca@...tu.xyz>
>> ---
>> Changes since RFC:
>> - address review comments from Jacek Anaszewski:
>>    - implement strobe_get op
>>    - implement timeout_set op
>>    - init v4l2_sd_cfg variable
>>    - remove init_data.devicename assignemnt
>>    - use devm_ version of led_classdev_flash_register_ext
>>    - release child_node in case of success
>>
>>   drivers/leds/Kconfig        |   9 ++
>>   drivers/leds/Makefile       |   1 +
>>   drivers/leds/leds-sgm3140.c | 260 ++++++++++++++++++++++++++++++++++++
>>   3 files changed, 270 insertions(+)
>>   create mode 100644 drivers/leds/leds-sgm3140.c
>>
>> diff --git a/drivers/leds/Kconfig b/drivers/leds/Kconfig
>> index 4b68520ac251..9206fc66799d 100644
>> --- a/drivers/leds/Kconfig
>> +++ b/drivers/leds/Kconfig
>> @@ -836,6 +836,15 @@ config LEDS_LM36274
>>         Say Y to enable the LM36274 LED driver for TI LMU devices.
>>         This supports the LED device LM36274.
>>   +config LEDS_SGM3140
>> +    tristate "LED support for the SGM3140"
>> +    depends on LEDS_CLASS_FLASH
>> +    depends on V4L2_FLASH_LED_CLASS || !V4L2_FLASH_LED_CLASS
> 
> What is the purpose of this?  Enable if V4L2_FLASH_LED_CLASS is enabled
> or if it is not enabled?
> 
> Seems to be a do nothing dependency in the Kconfig

See the patch: 58d1809b9d61 ("leds: fix aat1290 build errors")

and Documentation/kbuild/kconfig-language.rst,
chapter "Menu dependencies".

According to that ('!' <expr>)  boils down to (2-/expr/),
where

"An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
respectively for calculations)".

In a result:
- in case V4L2_FLASH_LED_CLASS=n it evaluates to 2,
- in case V4L2_FLASH_LED_CLASS=m it evaluates to 1,
- in case V4L2_FLASH_LED_CLASS=y it evaluates to 0

And (<expr> '||' <expr>) returns the result of max(/expr/, /expr/).

It allows to restrict LEDS_SGM3140 to m if CONFIG_V4L2_FLASH_LED_CLASS=m
(we will have max(1,1) then)
and ignore the dependency if CONFIG_V4L2_FLASH_LED_CLASS=n
(max(0,2)).

-- 
Best regards,
Jacek Anaszewski