linux-kernel - Re: [PATCH 2/2] rtc: tegra: Implement clock handling

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Date:   Thu, 19 Jan 2017 14:23:53 +0200
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     Thierry Reding <thierry.reding@...il.com>
CC:     Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Martin Michlmayr <tbm@...ius.com>,
        <rtc-linux@...glegroups.com>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] rtc: tegra: Implement clock handling

On Thu, Jan 12, 2017 at 05:07:43PM +0100, Thierry Reding wrote:
> From: Thierry Reding <treding@...dia.com>
> 
> Accessing the registers of the RTC block on Tegra requires the module
> clock to be enabled. This only works because the RTC module clock will
> be enabled by default during early boot. However, because the clock is
> unused, the CCF will disable it at late_init time. This causes the RTC
> to become unusable afterwards. This can easily be reproduced by trying
> to use the RTC:
> 
> 	$ hwclock --rtc /dev/rtc1
> 
> This will hang the system. I ran into this by following up on a report
> by Martin Michlmayr that reboot wasn't working on Tegra210 systems. It
> turns out that the rtc-tegra driver's ->shutdown() implementation will
> hang the CPU, because of the disabled clock, before the system can be
> rebooted.
> 
> What confused me for a while is that the same driver is used on prior
> Tegra generations where the hang can not be observed. However, as Peter
> De Schrijver pointed out, this is because on 32-bit Tegra chips the RTC
> clock is enabled by the tegra20_timer.c clocksource driver, which uses
> the RTC to provide a persistent clock. This code is never enabled on
> 64-bit Tegra because the persistent clock infrastructure does not exist
> of 64-bit ARM.

'on 64-bit ARM' I guess?

Other than that Acked-By Peter De Schrijver <pdeschrijver@...dia.com>

> 
> The proper fix for this is to add proper clock handling to the RTC
> driver in order to ensure that the clock is enabled when the driver
> requires it. All device trees contain the clock already, therefore
> no additional changes are required.
> 
> Cc: Peter De Schrijver <pdeschrijver@...dia.com>
> Reported-by: Martin Michlmayr <tbm@...ius.com>
> Signed-off-by: Thierry Reding <treding@...dia.com>
> ---
>  drivers/rtc/rtc-tegra.c | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/rtc/rtc-tegra.c b/drivers/rtc/rtc-tegra.c
> index 38e662ff1a70..d30d57b048d3 100644
> --- a/drivers/rtc/rtc-tegra.c
> +++ b/drivers/rtc/rtc-tegra.c
> @@ -18,6 +18,7 @@
>   * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
>   */
>  
> +#include <linux/clk.h>
>  #include <linux/delay.h>
>  #include <linux/init.h>
>  #include <linux/io.h>
> @@ -60,6 +61,7 @@ struct tegra_rtc_info {
>  	struct platform_device	*pdev;
>  	struct rtc_device	*rtc_dev;
>  	void __iomem		*rtc_base; /* NULL if not initialized. */
> +	struct clk		*clk;
>  	int			tegra_rtc_irq; /* alarm and periodic irq */
>  	spinlock_t		tegra_rtc_lock;
>  };
> @@ -327,6 +329,14 @@ static int __init tegra_rtc_probe(struct platform_device *pdev)
>  	if (info->tegra_rtc_irq <= 0)
>  		return -EBUSY;
>  
> +	info->clk = devm_clk_get(&pdev->dev, NULL);
> +	if (IS_ERR(info->clk))
> +		return PTR_ERR(info->clk);
> +
> +	ret = clk_prepare_enable(info->clk);
> +	if (ret < 0)
> +		return ret;
> +
>  	/* set context info. */
>  	info->pdev = pdev;
>  	spin_lock_init(&info->tegra_rtc_lock);
> @@ -347,7 +357,7 @@ static int __init tegra_rtc_probe(struct platform_device *pdev)
>  		ret = PTR_ERR(info->rtc_dev);
>  		dev_err(&pdev->dev, "Unable to register device (err=%d).\n",
>  			ret);
> -		return ret;
> +		goto disable_clk;
>  	}
>  
>  	ret = devm_request_irq(&pdev->dev, info->tegra_rtc_irq,
> @@ -357,12 +367,25 @@ static int __init tegra_rtc_probe(struct platform_device *pdev)
>  		dev_err(&pdev->dev,
>  			"Unable to request interrupt for device (err=%d).\n",
>  			ret);
> -		return ret;
> +		goto disable_clk;
>  	}
>  
>  	dev_notice(&pdev->dev, "Tegra internal Real Time Clock\n");
>  
>  	return 0;
> +
> +disable_clk:
> +	clk_disable_unprepare(info->clk);
> +	return ret;
> +}
> +
> +static int tegra_rtc_remove(struct platform_device *pdev)
> +{
> +	struct tegra_rtc_info *info = platform_get_drvdata(pdev);
> +
> +	clk_disable_unprepare(info->clk);
> +
> +	return 0;
>  }
>  
>  #ifdef CONFIG_PM_SLEEP
> @@ -414,6 +437,7 @@ static void tegra_rtc_shutdown(struct platform_device *pdev)
>  
>  MODULE_ALIAS("platform:tegra_rtc");
>  static struct platform_driver tegra_rtc_driver = {
> +	.remove		= tegra_rtc_remove,
>  	.shutdown	= tegra_rtc_shutdown,
>  	.driver		= {
>  		.name	= "tegra_rtc",
> -- 
> 2.11.0
>