linux-kernel - Re: [PATCH] pwm: pwm-imx27: Use 'dev' instead of dereferencing it repeatedly

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Message-ID: <20191002073319.tv55olneh6i6x4ir@pengutronix.de>
Date:   Wed, 2 Oct 2019 09:33:19 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Anson Huang' <Anson.Huang@....com>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "festevam@...il.com" <festevam@...il.com>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Linux-imx@....com" <Linux-imx@....com>
Subject: Re: [PATCH] pwm: pwm-imx27: Use 'dev' instead of dereferencing it
 repeatedly

On Tue, Sep 24, 2019 at 09:46:20AM +0000, David Laight wrote:
> From: Anson Huang
> > Sent: 24 September 2019 10:00
> > Add helper variable dev = &pdev->dev to simply the code.
> > 
> > Signed-off-by: Anson Huang <Anson.Huang@....com>
> > ---
> >  drivers/pwm/pwm-imx27.c | 13 +++++++------
> >  1 file changed, 7 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
> > index 434a351..3afee29 100644
> > --- a/drivers/pwm/pwm-imx27.c
> > +++ b/drivers/pwm/pwm-imx27.c
> > @@ -290,27 +290,28 @@ MODULE_DEVICE_TABLE(of, pwm_imx27_dt_ids);
> > 
> >  static int pwm_imx27_probe(struct platform_device *pdev)
> >  {
> > +	struct device *dev = &pdev->dev;
> >  	struct pwm_imx27_chip *imx;
> > 
> > -	imx = devm_kzalloc(&pdev->dev, sizeof(*imx), GFP_KERNEL);
> > +	imx = devm_kzalloc(dev, sizeof(*imx), GFP_KERNEL);
> >  	if (imx == NULL)
> >  		return -ENOMEM;
> > 
> >  	platform_set_drvdata(pdev, imx);
> > 
> > -	imx->clk_ipg = devm_clk_get(&pdev->dev, "ipg");
> > +	imx->clk_ipg = devm_clk_get(dev, "ipg");
> >  	if (IS_ERR(imx->clk_ipg)) {
> > -		dev_err(&pdev->dev, "getting ipg clock failed with %ld\n",
> > +		dev_err(dev, "getting ipg clock failed with %ld\n",
> >  				PTR_ERR(imx->clk_ipg));
> >  		return PTR_ERR(imx->clk_ipg);
> >  	}
> > 
> > -	imx->clk_per = devm_clk_get(&pdev->dev, "per");
> > +	imx->clk_per = devm_clk_get(dev, "per");
> >  	if (IS_ERR(imx->clk_per)) {
> >  		int ret = PTR_ERR(imx->clk_per);
> > 
> >  		if (ret != -EPROBE_DEFER)
> > -			dev_err(&pdev->dev,
> > +			dev_err(dev,
> >  				"failed to get peripheral clock: %d\n",
> >  				ret);
> 
> Hopefully the compiler will optimise this back otherwise you've added another
> local variable which may cause spilling to stack.

I wonder that in reply to this comment nobody actually tried. I just did
that and applying the patch doesn't change the resulting binary. (Tested
with gcc 7.3.1).

> For a setup function it probably doesn't matter, but in general it might
> have a small negative performance impact.
> 
> In any case this doesn't shorten any lines enough to remove line-wrap
> and using &pdev->dev is really one less variable to mentally track
> when reading the code.

On the other hand having a variable named "dev" is so usual that I
personally slightly prefer using it instead of &pdev->dev. So I think
(given there is no effect on the compiled result) this is really just
about personal taste and so to actually switch from one preference to
the other needs a better justification IMHO.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |