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

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Date:   Thu, 05 Jul 2018 22:56:40 +0200
From:   Janusz Krzysztofik <jmkrzyszt@...il.com>
To:     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 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