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

[Daniel Lezcano]

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@xxxxxxxxxx>:
> > On 03/12/2015 10:55 PM, Maxime Coquelin wrote:
> > > From: Maxime Coquelin <mcoquelin.stm32@xxxxxxxxx>
> > >
> > > This patch adds clocksource support for ARMv7-M's System timer,
> > > also known as SysTick.
> > >
> > > Signed-off-by: Maxime Coquelin <mcoquelin.stm32@xxxxxxxxx>
> >
> >
> > 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

--
 <http://www.linaro.org/> Linaro.org â Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/