linux-kernel - Re: [PATCH] gpiolib: Defer on non-DT find_chip_by

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Date:   Fri, 6 Jul 2018 10:03:53 +0100
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     Janusz Krzysztofik <jmkrzyszt@...il.com>,
        Lee Jones <lee.jones@...aro.org>
CC:     Boris Brezillon <boris.brezillon@...tlin.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        <linux-gpio@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Fabio Estevam <fabio.estevam@....com>,
        Phil Reid <preid@...ctromag.com.au>,
        Lucas Stach <l.stach@...gutronix.de>,
        Clemens Gruber <clemens.gruber@...ruber.com>,
        Peter Rosin <peda@...ntia.se>, <patches@...nsource.cirrus.com>,
        <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH] gpiolib: Defer on non-DT find_chip_by_name() failure

On 05/07/18 21:56, Janusz Krzysztofik wrote:
> On Thursday, July 5, 2018 7:50:37 AM CEST Lee Jones wrote:
>> On Wed, 04 Jul 2018, Janusz Krzysztofik wrote:
>>> On Tuesday, July 3, 2018 7:31:41 PM CEST Boris Brezillon wrote:
>>>> Hi Janusz,
>>>>
>>>> On Tue,  3 Jul 2018 19:26:35 +0200
>>>>
>>>> Janusz Krzysztofik <jmkrzyszt@...il.com> wrote:
>>>>> Avoid replication of error code conversion in non-DT GPIO consumers'
>>>>> code by returning -EPROBE_DEFER from gpiod_find() in case a chip
>>>>> identified by its label in a registered lookup table is not ready.
>>>>>
>>>>> See https://lkml.org/lkml/2018/5/30/176 for example case.
>>>>>
>>>>> Signed-off-by: Janusz Krzysztofik <jmkrzyszt@...il.com>
>>>>> ---
>>>>> If accepted, please add
>>>>>
>>>>> 	Suggested-by: Boris Brezillon <boris.brezillon@...tlin.com>
>>>>>
>>>>> if Boris doesn't mind.
>>>>>
>>>>> Thanks,
>>>>> Janusz
>>>>>
>>>>>   drivers/gpio/gpiolib.c | 13 ++++++++++---
>>>>>   1 file changed, 10 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
>>>>> index e11a3bb03820..15dc77c80328 100644
>>>>> --- a/drivers/gpio/gpiolib.c
>>>>> +++ b/drivers/gpio/gpiolib.c
>>>>> @@ -3639,9 +3639,16 @@ static struct gpio_desc *gpiod_find(struct
>>>>> device
>>>>> *dev, const char *con_id,>
>>>>>
>>>>>   		chip = find_chip_by_name(p->chip_label);
>>>>>   		
>>>>>   		if (!chip) {
>>>>>
>>>>> -			dev_err(dev, "cannot find GPIO chip %s\n",
>>>>> -				p->chip_label);
>>>>> -			return ERR_PTR(-ENODEV);
>>>>> +			/*
>>>>> +			 * As the lookup table indicates a chip with
>>>>> +			 * p->chip_label should exist, assume it may
>>>>> +			 * still appear latar and let the interested
>>>>>
>>>> 					^ later
>>>>>
>>>>> +			 * consumer be probed again or let the Deferred
>>>>> +			 * Probe infrastructure handle the error.
>>>>> +			 */
>>>>> +			dev_warn(dev, "cannot find GPIO chip %s, deferring\n",
>>>>> +				 p->chip_label);
>>>>> +			return ERR_PTR(-EPROBE_DEFER);
>>>>>
>>>>>   		}
>>>>>   		
>>>>>   		if (chip->ngpio <= p->chip_hwnum) {
>>>>
>>>> Looks good otherwise. Let's hope we're not breaking implementations
>>>> testing for -ENODEV...
>>>
>>> I've reviewed them all and found two which I think may be affected:
>>> - drivers/mfd/arizona-core.c,
>>> - drivers/i2c/busses/i2c-imx.c.
>>> As far as I can understand the code, both depend on error != -EPROBE_DEFER
>>> in order to continue in degraded mode. I'm adding their maintainers to
>>> the loop.
>>  From a quick glance, the -EPROBE_DEFER handing in Arizona Core appears
>> to be correct.  Would you mind explaining what your concerns are in
>> more detail please?
> 
> Hi
> 
> That's more about handling -ENODEV rather than -EPROBE_DEFER.
> 
> Before the change, if GPIO chip supposed to provide "reset" pin was not ready
> during  arizona_dev_init(), devm_gpiod_get() returned -ENODEV and device was
> initialized in degraded mode, i.e., with no control over the "reset" pin.
> After the change, gpiod_get() will return -EPROBE_DEFER in such case and
> arizona_dev_init() won't succeed in case the GPIO chip doesn't appear later
> for some reason.
> 
> Thanks,
> Januszz
> 
> 

The intention is that if the DT node is missing, the Arizona driver can run
using only soft reset, though there are limitations in that mode.
This should return -ENOENT so that the Arizona driver will continue without
a GPIO.

If the DT defines a GPIO it is effectively saying that this GPIO is required so
it is valid for the Arizona driver never to come up if the GPIO it is defined to
depend on doesn't come up.