linux-kernel - Re: [RFC PATCH 2/2] rtc: s3c: Add s3c_rtc_{enable/disable}_clk in s3c_rtc

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Date:	Tue, 05 Jul 2016 14:22:07 +0530
From:	Alim Akhtar <alim.akhtar@...sung.com>
To:	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org
Cc:	alexandre.belloni@...e-electrons.com, javier@....samsung.com,
	pankaj.dubey@...sung.com
Subject: Re: [RFC PATCH 2/2] rtc: s3c: Add s3c_rtc_{enable/disable}_clk in
 s3c_rtc_setfreq()



On 07/05/2016 01:46 PM, Krzysztof Kozlowski wrote:
> On 07/04/2016 01:03 PM, Alim Akhtar wrote:
>> As per code flow it is possible that s3c_rtc_setfreq() might get called
>> with rtc clock disabled and in set_freq we perform h/w registers read/write,
>> which might results in a kernel crash while probing rtc driver.
>> Below is one such case:
>> s3c_rtc_probe()
>>      clk_prepare_enable(info->rtc_clk) // rtc clock enabled
>>        s3c_rtc_gettime() // will enable clk if not done, and disable it upon exit
>>          s3c_rtc_setfreq() //then this will be called with clk disabled
>
> The indentation suggests levels of calls (chain) not sequence. This
> should be:
> s3c_rtc_probe()
>    clk_prepare_enable(info->rtc_clk) // rtc clock enabled
>    s3c_rtc_gettime() // will enable clk if not done, and disable it upon exit
>    s3c_rtc_setfreq() //then this will be called with clk disabled
>
>>
>> This patch take cares of such issue by adding s3c_rtc_{enable/disable}_clk in
>> s3c_rtc_setfreq().
>
> What I don't get is that you wrote "it is *possible* that
> s3c_rtc_setfreq() *might* get called". From my understanding this will
> happen always because src_rtc_gettime() always disables the clocks.
>
> Why it does not happen always?
>

Yes, you are right, it is always disabled when reaches s3c_rtc_setfreq().
And I observed a kernel crash while testing on exynos7 platform in 
s3c_rtc_setfreq() because clock was always disabled at this point. And 
this patches fixes this issue.


> Best regards,
> Krzysztof
>