linux-kernel - Re: [PATCHv3 3/3] clocksource: Add Freescale FlexTimer Module (FTM) timer support

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Message-ID: <5379CA04.2040801@linaro.org>
Date:	Mon, 19 May 2014 11:08:20 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	"Li.Xiubo@...escale.com" <Li.Xiubo@...escale.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Jingchang Lu <jingchang.lu@...escale.com>
Subject: Re: [PATCHv3 3/3] clocksource: Add Freescale FlexTimer Module (FTM)
 timer support

On 05/19/2014 04:26 AM, Li.Xiubo@...escale.com wrote:
>>> +#define FTM_CNTIN	0x4C
>>> +
>>> +static void __iomem *clksrc_base;
>>> +static void __iomem *clkevt_base;
>>> +static unsigned long peroidic_cyc;
>>> +static unsigned long ps;
>>> +bool big_endian;
>>> +
>>
>> Usually this is encaspulated in a structure.
>>
>> struct ftm_clock_device {
>> 	void __iomem *clksrc_base;
>> 	...
>> };
>>
>>
>>> +static inline u32 ftm_readl(void __iomem *addr)
>>> +{
>>> +	if (big_endian)
>>
>> I am not a big fan of addressing global variables in the functions, so
>> if you can pass the structure pointer around here and the other
>> functions instead that would be nice.
>>
>> Otherwise the patch sounds ok. Thanks for taking care of encapsulating
>> well the functions and commenting the code.
>>
>
> Yes, I did think so.
>
> But some callbacks like ：
> + static u64 ftm_read_sched_clock(void)
> + {
> +         return ftm_readl(clksrc_base + FTM_CNT);
> + }
>
> Used by :
> + sched_clock_register(ftm_read_sched_clock,....);
>
> If they are encapsulated in a structure, and should the struct instance
> be one global instance too ? I'm doubting whether will this make sense ?

Actually, I plan in a near future to consolidate the code and factor out 
some parts with a common structure across the different drivers. So even 
if you address the base@ with the global instance but pass around the 
structure as parameter, that will be ok because that will be less 
modifications in the future. It is not a strong requirement, just put in 
place some encapsulation to make the life easier for after.

>>> +static int __init ftm_calc_closest_round_cyc(unsigned long freq)
>>> +{
>>> +	ps = 0;
>>> +
>>> +	do {
>>> +		peroidic_cyc = DIV_ROUND_CLOSEST(freq, HZ * (1 << ps++));
>>> +	} while (peroidic_cyc > 0xFFFF);
>>> +
>>> +	if (ps > 7) {
>>> +		pr_err("ftm: the max prescaler is %lu > 7\n", ps);
>>> +		return -EINVAL;
>>> +	}
>>> +
>>
>> Can you explain how this error can happen ?
>>
>
> Yes, the hardware limitation of the 'ps' is 0~7, and the counter register
> Is only using the lower 16 bits.
> If the 'freq' value is too big here, then the periodic_cyc may exceed 0xFFFF.
>
> Or should I add some comment here ?

Yes, a comment will be welcome.

Thanks
   -- Daniel

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