linux-kernel - Re: [PATCH] rtc: armada38x: add __ro_after_init to armada38x_rtc

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Message-ID: <alpine.DEB.2.20.1701041155380.3073@hadrien>
Date:   Wed, 4 Jan 2017 11:57:00 +0100 (CET)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
cc:     Kees Cook <keescook@...omium.org>, andrew@...n.ch,
        Jason Cooper <jason@...edaemon.net>,
        rtc-linux@...glegroups.com, a.zummo@...ertech.it,
        LKML <linux-kernel@...r.kernel.org>,
        Julia Lawall <julia.lawall@...6.fr>,
        alexandre.belloni@...e-electrons.com,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        gregory.clement@...e-electrons.com,
        Bhumika Goyal <bhumirks@...il.com>,
        sebastian.hesselbarth@...il.com
Subject: Re: [PATCH] rtc: armada38x: add __ro_after_init to
 armada38x_rtc_ops



On Tue, 3 Jan 2017, Russell King - ARM Linux wrote:

> On Tue, Jan 03, 2017 at 09:31:18PM +0000, Russell King - ARM Linux wrote:
> > On Tue, Jan 03, 2017 at 01:18:29PM -0800, Kees Cook wrote:
> > > On Mon, Jan 2, 2017 at 6:06 AM, Russell King - ARM Linux
> > > <linux@...linux.org.uk> wrote:
> > > > On Mon, Dec 26, 2016 at 05:01:02PM +0530, Bhumika Goyal wrote:
> > > >> The object armada38x_rtc_ops of type rtc_class_ops structure is not
> > > >> modified after getting initialized by armada38x_rtc_probe. Apart from
> > > >> getting referenced in init it is also passed as an argument to the function
> > > >> devm_rtc_device_register but this argument is of type const struct
> > > >> rtc_class_ops *. Therefore add __ro_after_init to its declaration.
> > > >
> > > > What I'd prefer here is for the structure to be duplicated, with one
> > > > copy having the alarm methods and one which does not.  Both can then
> > > > be made "const" (so placed into the read-only section at link time)
> > > > and the probe function select between the two.
> > > >
> > > > I think that's a cleaner and better solution, even though it's
> > > > slightly larger.
> > > >
> > > > I'm not a fan of __ro_after_init being used where other solutions are
> > > > possible.
> > >
> > > Can the pointer that points to the struct rtc_class_ops be made ro_after_init?
> >
> > It's passed into the RTC core code, and probably stored in some dynamically
> > allocated object, so probably no.  It's the same class of problem as every
> > file_operations pointer in the kernel, or the thousand other operations
> > structure pointers that a running kernel has.

I'm not sure to understand the question and the response.  A quick check
with grep suggests that most rtc_class_ops pointers are already const.
There seem to be just some instances in specific drivers that are not.

julia

>
> For the elimination of doubt, this is what I meant in my original email.
> As you can see, there's nothing to be marked as __ro_after_init anymore.
>
>  drivers/rtc/rtc-armada38x.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/rtc/rtc-armada38x.c b/drivers/rtc/rtc-armada38x.c
> index 9a3f2a6f512e..a4166ccfce36 100644
> --- a/drivers/rtc/rtc-armada38x.c
> +++ b/drivers/rtc/rtc-armada38x.c
> @@ -202,7 +202,7 @@ static irqreturn_t armada38x_rtc_alarm_irq(int irq, void *data)
>  	return IRQ_HANDLED;
>  }
>
> -static struct rtc_class_ops armada38x_rtc_ops = {
> +static const struct rtc_class_ops armada38x_rtc_ops = {
>  	.read_time = armada38x_rtc_read_time,
>  	.set_time = armada38x_rtc_set_time,
>  	.read_alarm = armada38x_rtc_read_alarm,
> @@ -210,8 +210,15 @@ static struct rtc_class_ops armada38x_rtc_ops = {
>  	.alarm_irq_enable = armada38x_rtc_alarm_irq_enable,
>  };
>
> +static const struct rtc_class_ops armada38x_rtc_ops_noirq = {
> +	.read_time = armada38x_rtc_read_time,
> +	.set_time = armada38x_rtc_set_time,
> +	.read_alarm = armada38x_rtc_read_alarm,
> +};
> +
>  static __init int armada38x_rtc_probe(struct platform_device *pdev)
>  {
> +	const struct rtc_class_ops *ops;
>  	struct resource *res;
>  	struct armada38x_rtc *rtc;
>  	int ret;
> @@ -242,19 +249,22 @@ static __init int armada38x_rtc_probe(struct platform_device *pdev)
>  				0, pdev->name, rtc) < 0) {
>  		dev_warn(&pdev->dev, "Interrupt not available.\n");
>  		rtc->irq = -1;
> +	}
> +	platform_set_drvdata(pdev, rtc);
> +
> +	if (rtc->irq != -1) {
> +		device_init_wakeup(&pdev->dev, 1);
> +		ops = &armada38x_rtc_ops;
> +	} else {
>  		/*
>  		 * If there is no interrupt available then we can't
>  		 * use the alarm
>  		 */
> -		armada38x_rtc_ops.set_alarm = NULL;
> -		armada38x_rtc_ops.alarm_irq_enable = NULL;
> +		ops = &armada38x_rtc_ops_noirq;
>  	}
> -	platform_set_drvdata(pdev, rtc);
> -	if (rtc->irq != -1)
> -		device_init_wakeup(&pdev->dev, 1);
>
>  	rtc->rtc_dev = devm_rtc_device_register(&pdev->dev, pdev->name,
> -					&armada38x_rtc_ops, THIS_MODULE);
> +						ops, THIS_MODULE);
>  	if (IS_ERR(rtc->rtc_dev)) {
>  		ret = PTR_ERR(rtc->rtc_dev);
>  		dev_err(&pdev->dev, "Failed to register RTC device: %d\n", ret);
>
> --
> RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
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>