linux-kernel - Re: Why do we need reset_control_get

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Date:	Mon, 08 Aug 2016 18:39:36 +0200
From:	Philipp Zabel <p.zabel@...gutronix.de>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	Axel Lin <axel.lin@...ics.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Lee Jones <lee.jones@...aro.org>
Subject: Re: Why do we need reset_control_get_optional() ?

Am Freitag, den 05.08.2016, 17:50 +0200 schrieb Arnd Bergmann:
> On Thursday, July 28, 2016 1:00:49 PM CEST Philipp Zabel wrote:
> > Am Donnerstag, den 28.07.2016, 19:52 +0900 schrieb Masahiro Yamada:
> 
> > > > In my experimental patch, I make the _optional functions
> > > > return NULL if no "resets" property is provided but return
> > > > an error if there are reset lines but the subsystem is
> > > > disabled, i.e. an optional reset must be used if it's in the
> > > > DT, but can be ignored otherwise.
> > > 
> > > I do not like this idea.
> > > 
> > > reset_control_get() (or variants) should not return NULL, it is ambiguous.
> > > It should return ERR_PTR(-ENOENT) if no "resets" property.
> > > 
> > > I only want two types for functions that return a pointer.
> > > 
> > > [1] return a valid pointer on success, or return NULL on failure
> > >     (for example, kmalloc())
> > > [2] return a valid pointer on success, or return error pointer on failure
> > >    (many of _register() functions)
> > > 
> > > Mixing [1] and [2] will be a mess.
> 
> Ah, right. I was thinking only of the case where the reset subsystem
> is completely disabled here, so returning NULL could be considered
> a valid return code that can in turn be passed into the other
> functions.
> 
> However, I agree that returning NULL as a valid result from
> ..._get_optional() would be bad style, so let's drop my idea
> there.
> 
> > I too would prefer to keep that as-is. The reset_control_get_optional
> > stub could return -ENOENT if there is no resets device tree property.
> 
> Now I'm also confused about what we really need
> reset_control_get_optional() for, and which error codes the callers
> are supposed to check.
> 
> This is the matrix I think you mean for _get_optional:
> 
[...]
> CONFIG_RESET_CONTROLLER=n, dt entry present: -EOPNOTSUPP
> CONFIG_RESET_CONTROLLER=n, dt entry missing: -ENOENT

^^ I didn't consider this distiction.

> Is this what you had in mind? If so, what is the value of the
> added runtime warning for reset_control_get? Any caller of that
> function would already check for errors, the only difference
> I see is that callers of _optional can ignore -ENOENT.

My initial motivation was to make it as hard as possible to misconfigure
the kernel, which is why I initially didn't want stubs for the
non-optional variant. Of course that would cause build failures and/or
reduced compile test coverage, so we added the stubs and the warning to
make it obvious when a misconfigured kernel is running: on a kernel with
RESET_CONTROLLER=n drivers that use reset_control_get are expected to
build, but they are not expected to work. I suppose the same is the case
for _optional, if the dt entry is present, so maybe we should drop
reset_control_get_optional and add always a warning in case of
-EOPNOTSUPP.
I don't want all drivers to have to differentiate between -EOPNOTSUPP
and -ENOENT error codes, only current reset_control_get_optional users
have to do that.

regards
Philipp