Date:   Mon, 04 Jun 2018 18:48:08 +0200
From:   Janusz Krzysztofik <jmkrzyszt@...il.com>
To:     Boris Brezillon <boris.brezillon@...tlin.com>
Cc:     Richard Weinberger <richard@....at>,
        H Hartley Sweeten <hsweeten@...ionengravers.com>,
        Tony Lindgren <tony@...mide.com>, linux-kernel@...r.kernel.org,
        Marek Vasut <marek.vasut@...il.com>,
        Krzysztof Halasa <khalasa@...p.pl>,
        Shreeya Patel <shreeya.patel23498@...il.com>,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        linux-mtd@...ts.infradead.org
Subject: Re: [PATCH 5/6 v2] mtd: rawnand: ams-delta: use GPIO lookup table

On Monday, June 4, 2018 11:55:43 AM CEST Boris Brezillon wrote:
> On Wed, 30 May 2018 22:39:03 +0200
> 
> Janusz Krzysztofik <jmkrzyszt@...il.com> wrote:
> > On Wednesday, May 30, 2018 7:52:20 PM CEST Boris Brezillon wrote:
> > > On Wed, 30 May 2018 19:43:09 +0200
> > > 
> > > Janusz Krzysztofik <jmkrzyszt@...il.com> wrote:
> > > > On Wednesday, May 30, 2018 11:05:00 AM CEST Boris Brezillon wrote:
> > > > > Hi Janusz,
> > > > 
> > > > Hi Boris,
> > > > 
> > > > > On Sat, 26 May 2018 00:20:45 +0200
> > > > > 
> > > > > Janusz Krzysztofik <jmkrzyszt@...il.com> wrote:
> > > > > > ...
> > > > > > Changes since v1:
> > > > > > - fix handling of devm_gpiod_get_optional() return values - thanks
> > > > > > to
> > > > > > 
> > > > > >   Andy Shevchenko.
> > > > > 
> > > > > Can you put the changelog after the "---" separator so that it does
> > > > > not
> > > > > appear in the final commit message?
> > > > 
> > > > Yes, sure, sorry for that.
> > > > 
> > > > > > +err_gpiod:
> > > > > > +	if (err == -ENODEV || err == -ENOENT)
> > > > > > +		err = -EPROBE_DEFER;
> > > > > 
> > > > > Hm, isn't it better to make gpiod_find() return
> > > > > ERR_PTR(-EPROBE_DEFER)
> > > > > here [1]? At least, ENOENT should not be turned into EPROBE_DEFER,
> > > > > because it's returned when there's no entry matching the requested
> > > > > gpio
> > > > > in the lookup table, and deferring the probe won't solve this
> > > > > problem.
> > > > 
> > > > ENOENT is also returned when no matching lookup table is found. That
> > > > may
> > > > happen if consumer dev_name stored in the table differs from dev_name
> > > > assigned to the consumer by its bus, the platform bus in this case.
> > > > For
> > > > that reason I think the consumer dev_name should be initialized in the
> > > > table after the device is registered, when its actual dev_name can be
> > > > obtained. If that device registration happens after the driver is
> > > > already
> > > > registered, e.g., at late_initcall, the device is probed before its
> > > > lookup table is ready. For that reason returning EPROBE_DEFER seems
> > > > better to me even in the ENOENT case.
> > > 
> > > Sorry, I don't get it. Aren't GPIO lookup tables supposed to be declared
> > > in board files, especially if the GPIO is used by a platform device?
> > > When would you have a lookup table registered later in the init/boot
> > > process?
> > 
> > When e.g. I'd like to register my GPIO consumer platform device at
> > late_initcall for some reason, and I'm not sure what exact dev_name my
> > consomer will be registered with by the platform bus.
> 
> You should know the name before the device is registered.

What if I use PLATFORM_DEVID_AUTO?

For other cases, if bus specific names of devices were supposed to be known 
before registration, bus drivers should export functions returning those names 
from initialized bus specific device structures or their components while they 
don't. Under such circumstances, we end up hardcoding device names based on 
our knowledge of bus internals if we need to specify them in advance, and 
those internals are not guaranteed to never change.

> > In that case I think I
> > should assign dev_name to the lookup table after the consumer device is
> > registered and its exact dev_name can be obtained, then register the
> > table,
> 
> I'm pretty sure it's not supposed to work like that. Resources attached
> to a device should be defined before the device is registered, not
> after,

What do you mean by resources attached to a device? I don't think we should 
consider GPIO lookup tables as consumer device resources. Those tables are 
registered separately from consumer device registration and I know of no 
requirement for registering them in advance. Maybe I'm missing something.

Let's have a look at regulators. There are no separately registered regulator 
lookup tables, instead regulator consumer supply tables are attached to bus 
specific device structures of regulator devices, not their consumers, hence 
registered together with providers, not consumers. Will you still call those 
tables 'resources attached to' consumers? 

As far as I can see, regulator_get() never returns -EINVAL, only -ENODEV or -
EPROBE_DEFER. However, gpiod_get() can also return -EINVAL. Maybe it 
shouldn't, but it does, and I'm just trying to adopt to that in order to not 
break a driver I'm trying to update. 

> simply because when you call platform_device_register(), the
> device might be directly bind to the driver before the
> platform_device_register() calls return, and the driver will fail to
> probe the device if it doesn't find the GPIO it needs.

That's exactly the case I'm talking about, but my conclusion is different: the 
driver should fail softly so the device is probed again later, as long as I'm 
not wrong the no requirement exists for registering GPIO lookup tables before 
related consumers are registered.

If I' missing something or you are still not convinced, I'll try to resolve 
issues with the device I see in a different way and submit a new patch that 
hopefully matches your requirements.

Thanks,
Janusz

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