linux-kernel - Re: [PATCH 2/3] pinctrl: at91: initialize config parameter to 0

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Date:	Mon, 09 Dec 2013 10:55:26 +0100
From:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Linus Walleij <linus.walleij@...aro.org>
CC:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/3] pinctrl: at91: initialize config parameter to 0

Hi,

On 09/12/2013 09:24, Nicolas Ferre wrote:
> On 07/12/2013 14:08, Alexandre Belloni :
>> When passing a not initialized config parameter, at91_pinconf_get()
>> would return
>> a bogus value. Fix that by initializing it to zero before using it.
>>
>> Signed-off-by: Alexandre Belloni <alexandre.belloni@...e-electrons.com>
>> ---
>>   drivers/pinctrl/pinctrl-at91.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pinctrl/pinctrl-at91.c
>> b/drivers/pinctrl/pinctrl-at91.c
>> index 6446dc804aa7..b0b78f3468ae 100644
>> --- a/drivers/pinctrl/pinctrl-at91.c
>> +++ b/drivers/pinctrl/pinctrl-at91.c
>> @@ -722,7 +722,8 @@ static int at91_pinconf_get(struct pinctrl_dev
>> *pctldev,
>>       unsigned pin;
>>       int div;
>>
>> -    dev_dbg(info->dev, "%s:%d, pin_id=%d, config=0x%lx", __func__,
>> __LINE__, pin_id, *config);
>> +    *config = 0;
>> +    dev_dbg(info->dev, "%s:%d, pin_id=%d", __func__, __LINE__, pin_id);
>>       pio = pin_to_controller(info, pin_to_bank(pin_id));
>>       pin = pin_id % MAX_NB_GPIO_PER_BANK;
>
> Beyond this patch, I must say that I am puzzled by this function.
>
> What I read from the prototype documentation and what I see in
> different implementations is different...
>
> Linus, can we have a review of this function because it seems not in
> line with what is used for u300 (but on the other hand looks like the
> what is returned by pinctrl-exynos5440.c driver for example).
>
> What would be the consequences if we change this function's behavior:
> I mean use of -EINVAL for pin configuration "available but disabled"
> as said in include/linux/pinctrl/pinconf.h?
>

>From what I understand, it doesn't really matter because that function
is never used. I guess the best implementation is the tegra one ;)

It is only called in one specific case:
 - you have ops->is_generic == true (that is only true for a few
implmentations)
 - and you are dumping the pinconf state using debugfs

I'm actually wondering if the checks for the ops->pin_config_get are not
a bit overkill. We check for that function in:
 - pinconf_check_ops()
 - before calling it in pin_config_get_for_pin() which is only used
once, in the same path : dump using debugfs and having ops->is_generic
== true
 - in pinconf_pins_show() which is the function calling also in the same
path

What I would do is:
 - remove all the checks in pinconf_check_ops() and pinconf_pins_show()
so that people are not pressured to implement a function that is simply
never used.
 - modify pin_config_get_for_pin() by removing the dev_err() call and
returning -ENOTSUPP instead of -EINVAL (it doesn't change the behaviour
but I feel -ENOTSUPP is more appropriate)

I have a patch ready but I can't test it as I don't own any of the
is_generic platforms.

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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