Re: [PATCH v3 04/15] clocksource: Add ARM System timer driver

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 03/26/2015 09:19 PM, Maxime Coquelin wrote:
> Hi Daniel,
>
>    Thanks for the review. Please find my answers below.
>
> 2015-03-26 10:50 GMT+01:00 Daniel Lezcano <daniel.lezcano@linaro.org>:
>> On 03/12/2015 10:55 PM, Maxime Coquelin wrote:
>>>
>>> From: Maxime Coquelin <mcoquelin.stm32@gmail.com>
>>>
>>> This patch adds clocksource support for ARMv7-M's System timer,
>>> also known as SysTick.
>>>
>>> Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
>>
>>
>> Hi Maxime,
>>
>> the driver looks good. Three comments below.
>>
>>    -- Daniel
>>
>>

[ ... ]

>>> +static void __init system_timer_of_register(struct device_node *np)
>>> +{
>>> +       struct clk *clk;
>>> +       void __iomem *base;
>>> +       u32 rate = 0;
>>> +       int ret;
>>> +
>>> +       base = of_iomap(np, 0);
>>> +       if (!base) {
>>> +               pr_warn("system-timer: invalid base address\n");
>>> +               return;
>>> +       }
>>> +
>>> +       clk = of_clk_get(np, 0);
>>> +       if (!IS_ERR(clk)) {
>>> +               ret = clk_prepare_enable(clk);
>>> +               if (ret) {
>>> +                       clk_put(clk);
>>> +                       goto out_unmap;
>>> +               }
>>> +
>>> +               rate = clk_get_rate(clk);
>>> +       }
>>> +
>>> +       /* If no clock found, try to get clock-frequency property */
>>> +       if (!rate) {
>>> +               ret = of_property_read_u32(np, "clock-frequency", &rate);
>>> +               if (ret)
>>> +                       goto out_unmap;
>>
>>
>> Shouldn't be 'goto out_clk_disable' ?
>
> No, because I assumed !rate means we failed to get the clock.
> Actually, clk_get_rate could return 0, so relying on rate value is not safe.
>
> I propose to get clock-frequency property if IS_ERR(clk).
>
> Is it fine for you?

Why not invert the conditions ? If the 'clock-frequency' is specified in 
the DT then it overrides the clk_get_rate(). So the resulting code will be:

ret = of_property_read_u32(np, "clock-frequency", &rate);
if (ret) {
	clk = of_clk_get(np, 0);
	if (IS_ERR(clk))
		goto out_unmap;

	ret = clk_prepare_enable(clk);
	if (ret)
		goto out_clk_put;

	rate = clk_get_rate(clk);
	if (!rate)
		goto out_clk_unprepare;
}



>>> +       }
>>> +
>>> +       writel_relaxed(SYSTICK_LOAD_RELOAD_MASK, base + SYST_RVR);
>>> +       writel_relaxed(SYST_CSR_ENABLE, base + SYST_CSR);
>>> +
>>> +       ret = clocksource_mmio_init(base + SYST_CVR, "arm_system_timer",
>>> rate,
>>> +                       200, 24, clocksource_mmio_readl_down);
>>> +       if (ret) {
>>> +               pr_err("failed to init clocksource (%d)\n", ret);
>>> +               goto out_clk_disable;
>>> +       }
>>> +
>>> +       pr_info("ARM System timer initialized as clocksource\n");
>>> +
>>> +       return;
>>> +
>>> +out_clk_disable:
>>> +       if (!IS_ERR(clk))
>>
>>
>> Why do you need this check ?
>
> To handle the case were no clock was found, but a clk-frequency value
> was provided.
>
>>
>> It isn't missing a clk_put ?
>
> Right, thanks for spotting this.
>
> I wonder if it makes sense to implement the error path.
> If we fail to initialize the clocksource, the system will be unusable.
>
> Maybe I should just perform a BUG_ON() in the error cases, as most of
> the other clocksource drivers do.
> What is your view?

I prefer to not BUG_ON in the init functions because it already happen 
that drivers were bugging at init time and when a driver was reused on 
another platform with several timers available, the board was not able 
to boot because one timer was not used, hence not defined in the DT. I 
don't know if that could be the case for this platform but I prefer to 
keep thing going smoothly and return from init even if that lead to a 
kernel hang. Of course, the errors must be displayed (pr_warn, pr_err, 
pr_notice, etc ...).

>>
>>> +               clk_disable_unprepare(clk);
>>> +out_unmap:
>>> +       iounmap(base);
>>> +       WARN(ret, "ARM System timer register failed (%d)\n", ret);

pr_warn

Thanks

   -- Daniel

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