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Date:   Sun, 20 May 2018 21:27:05 +0200
From:   Ladislav Michl <ladis@linux-mips.org>
To:     Janusz Krzysztofik <jmkrzyszt@gmail.com>
Cc:     Andy Shevchenko <andy.shevchenko@gmail.com>,
        linux-fbdev@vger.kernel.org,
        ALSA Development Mailing List <alsa-devel@alsa-project.org>,
        Aaro Koskinen <aaro.koskinen@iki.fi>,
        Tony Lindgren <tony@atomide.com>,
        Richard Weinberger <richard@nod.at>,
        Mark Brown <broonie@kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Liam Girdwood <lgirdwood@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Ujfalusi <peter.ujfalusi@ti.com>,
        Boris Brezillon <boris.brezillon@bootlin.com>,
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        "open list:MEMORY TECHNOLOGY..." <linux-mtd@lists.infradead.org>,
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Subject: Re: [alsa-devel] [PATCH 5/6] mtd: rawnand: ams-delta: use GPIO
 lookup table
Message-ID: <20180520192705.GA12883@lenoch>
References: <20180518210954.29044-1-jmkrzyszt@gmail.com>
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On Sat, May 19, 2018 at 11:55:51PM +0200, Janusz Krzysztofik wrote:
> On Saturday, May 19, 2018 8:00:38 PM CEST Andy Shevchenko wrote:
> > On Sat, May 19, 2018 at 2:15 AM, Janusz Krzysztofik <jmkrzyszt@gmail.com> 
> wrote:
> > > On Friday, May 18, 2018 11:21:14 PM CEST Andy Shevchenko wrote:
> > >> On Sat, May 19, 2018 at 12:09 AM, Janusz Krzysztofik
> > >> 
> > >> <jmkrzyszt@gmail.com> wrote:
> > >> > +       gpiod_rdy = devm_gpiod_get_optional(&pdev->dev, "rdy",
> > >> > GPIOD_IN);
> > >> > +       if (!IS_ERR_OR_NULL(gpiod_rdy)) {
> > >> 
> > >> So, is it optional or not at the end?
> > >> If it is, why do we check for NULL?
> > > 
> > > As far as I can understand, nand_chip->dev_ready() callback is optional.
> > > That's why I decided to use the _optional variant of devm_gpiod_get(). In
> > > case of ams-delta, the dev_ready() callback depends on availability of
> > > the 'rdy' GPIO pin. As a consequence, I'm checking for both NULL and ERR
> > > in order to decide if dev_ready() will be supported.
> > > 
> > > I can pretty well replace it with the standard form and check for ERR only
> > > if the purpose of the _optional form is different.
> > 
> > NULL check in practice discards the _optional part of gpiod_get(). So,
> > either you use non-optional variant and decide how to handle an
> > errors, or user _optional w/o NULL check.
> 
> OK, I'm going to use something like the below while submitting v2:
> 
> -	gpiod_rdy = devm_gpiod_get_optional(&pdev->dev, "rdy", GPIOD_IN);
> -	if (!IS_ERR_OR_NULL(gpiod_rdy)) {
> -		this->dev_ready = ams_delta_nand_ready;
> -	} else {
> -		this->dev_ready = NULL;
> -		pr_notice("Couldn't request gpio for Delta NAND ready.\n");
> +	priv->gpiod_rdy = devm_gpiod_get_optional(&pdev->dev, "rdy",
> +						  GPIOD_IN);
> +	if (IS_ERR(priv->gpiod_rdy)) {
> +		err = PTR_ERR(priv->gpiod_nwp);
??? --------------------------------^^^^^^^^^
> +		dev_warn(&pdev->dev, "RDY GPIO request failed (%d)\n", err);
> +		goto err_gpiod;

Driver will just use worst case delay instead of RDY signal, so this
is perhaps too strict. I will work with degraded performance.

	ladis

>  	}
>  
> +	if (priv->gpiod_rdy)
> +		this->dev_ready = ams_delta_nand_ready;
> 
> > 
> > >> > +err_gpiod:
> > >> > +       if (err == -ENODEV || err == -ENOENT)
> > >> > +               err = -EPROBE_DEFER;
> > >> 
> > >> Hmm...
> > > 
> > > Amstrad Delta uses gpio-mmio driver. Unfortunatelty that driver is not
> > > availble before device init phase, unlike other crucial GPIO drivers which
> > > are initialized earlier, e.g. during the postcore or at latetst the
> > > subsys phase. Hence, devices which depend on GPIO pins provided by
> > > gpio-mmio must either be declared late or fail softly so they get another
> > > chance of being probed succesfully.
> > > 
> > > I thought of replacing the gpio-mmio platform driver with bgpio functions
> > > it exports but for now I haven't implemented it, not even shared the
> > > idea.
> > > 
> > > Does it really hurt to return -EPROBE_DEFER if a GPIO pin can't be
> > > obtained?
> > I'm only concerned if it would be an infinite defer in the case when
> > driver will never appear.
> > But I don't remember the details.
> 
> Deferred probes are handled effectively during late_initcall, no risk of 
> infinite defer, see drivers/base/dd.c for details.
> 
> Thanks,
> Janusz
> 
> 
> 
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