linux-kernel - Re: [PATCH 1/2 (v3)] leds-bcm6328: Reuse bcm6328_led_set() instead of copying its functionality

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Message-id: <5652E84B.1060804@samsung.com>
Date:	Mon, 23 Nov 2015 11:19:55 +0100
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Simon Arlott <simon@...e.lp0.eu>
Cc:	Álvaro Fernández Rojas <noltari@...il.com>,
	Jonas Gorski <jogo@...nwrt.org>,
	Richard Purdie <rpurdie@...ys.net>, linux-leds@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2 (v3)] leds-bcm6328: Reuse bcm6328_led_set() instead of
 copying its functionality

On 11/22/2015 09:40 PM, Simon Arlott wrote:
> When ensuring a consistent initial LED state in bcm6328_led (as they may
> be blinking instead of on/off), the LED register is set using an inverted
> copy of bcm6328_led_set(). To avoid further errors relating to active low
> handling, call this function directly instead.
>
> As bcm6328_led_set() expects to acquire the spinlock, call it after
> unlocking. There is no need to hold the spinlock between reading the
> current value and setting it again because the LED device has not yet
> been registered.
>
> Signed-off-by: Simon Arlott <simon@...e.lp0.eu>
> ---
> On 17/11/15 08:15, Jacek Anaszewski wrote:
>> On 11/17/2015 09:06 AM, Jacek Anaszewski wrote:
>>> On 11/17/2015 08:42 AM, Simon Arlott wrote:
>>>> On 16/11/15 21:33, Álvaro Fernández Rojas wrote:
>>>>> Still wrong, you are setting the led value after unlocking the spinlock.
>>>>
>>>> I have to unlock it because bcm6328_led_set() locks that spinlock.
>>>
>>> Commit message from the first version of the patch justified that
>>> properly. It should be preserved in the final patch:
>>>
>>> As bcm6328_led_set() expects to acquire the spinlock, narrow the locking
>>> to only cover reading of the current state. There is no need to hold the
>>> spinlock between reading the current value and setting it again because
>>> the LED device has not yet been registered.
>>
>> Hmm, if so, then spin_lock in bcm6328_led isn't needed at all, as it
>> is guaranteed that no concurrent process will be executing this
>> function.
>
> No, it's still required because it has to protect the read/modify/write
> for all the other LED devices that use the same register.

Right, other already registered LED class devices can interfere.

>   drivers/leds/leds-bcm6328.c | 8 ++------
>   1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/leds/leds-bcm6328.c b/drivers/leds/leds-bcm6328.c
> index c7ea5c6..95d0cf9 100644
> --- a/drivers/leds/leds-bcm6328.c
> +++ b/drivers/leds/leds-bcm6328.c
> @@ -314,14 +314,10 @@ static int bcm6328_led(struct device *dev, struct device_node *nc, u32 reg,
>   	} else {
>   		led->cdev.brightness = LED_OFF;
>   	}
> -
> -	if ((led->active_low && led->cdev.brightness == LED_FULL) ||
> -	    (!led->active_low && led->cdev.brightness == LED_OFF))
> -		bcm6328_led_mode(led, BCM6328_LED_MODE_ON);
> -	else
> -		bcm6328_led_mode(led, BCM6328_LED_MODE_OFF);
>   	spin_unlock_irqrestore(lock, flags);
>
> +	bcm6328_led_set(&led->cdev, led->cdev.brightness);
> +
>   	led->cdev.brightness_set = bcm6328_led_set;
>   	led->cdev.blink_set = bcm6328_blink_set;
>
>


-- 
Best Regards,
Jacek Anaszewski
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