linux-kernel - Re: [PATCH v2 5/6] gpiolib: consolidate GPIO lookups

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Date:   Thu, 10 Nov 2022 22:10:36 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        linux-acpi@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/6] gpiolib: consolidate GPIO lookups

On Thu, Nov 10, 2022 at 09:21:59AM -0800, Dmitry Torokhov wrote:
> On Thu, Nov 10, 2022 at 03:42:40PM +0200, Andy Shevchenko wrote:
> > On Wed, Nov 09, 2022 at 11:00:29AM -0800, Dmitry Torokhov wrote:
> > > On Wed, Nov 09, 2022 at 01:25:06PM +0200, Andy Shevchenko wrote:
> > > > On Tue, Nov 08, 2022 at 04:26:50PM -0800, Dmitry Torokhov wrote:

...

> > > > > +	if (!IS_ERR_OR_NULL(fwnode))
> > > > 
> > > > I think this is superfluous check.
> > > > 
> > > > Now in the form of this series, you have only a single dev_dbg() that tries to
> > > > dereference it. Do we really need to have it there, since every branch has its
> > > > own dev_dbg() anyway?
> > > 
> > > As I mentioned, I like to keep this check to show the reader that we
> > > should only descend into gpiod_find_by_fwnode() if we have a valid
> > > fwnode. It is less about code generation and more about the intent.
> > 
> > Yes, but if fwnode is not found, we have a next check for that.
> 
> No, the check you are talking about is for the GPIO not being located.
> It does not have anything to do with fwnode validity. You are relying on
> intimate knowledge of gpiod_find_by_fwnode() implementation and the fact
> that in the current form it will withstand ERR_PTR-encoded or NULL
> fwnode.
> 
> I want to have the source code so clear in its intent so that I can be
> woken up in the middle of the night with a huge hangover and still be
> able to tell how it is supposed to behave.

As you said let's leave it to Bart and Linus.

> > I really don't
> > think we lose anything by dropping the check and gaining the code generation as
> > a side effect.
> 
> This is cold path, happening only on startup. I am not saying that we
> want to make it slow unnecessarily, but a condition branch that might
> even get optimized out is not something we should be concerned here.

Agree, that's why I called it "side effect".

-- 
With Best Regards,
Andy Shevchenko