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

[Maxime Coquelin <mcoquelin.stm32@xxxxxxxxx>]

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
>
>
>> ---
>>   drivers/clocksource/Kconfig          |  7 ++++
>>   drivers/clocksource/Makefile         |  1 +
>>   drivers/clocksource/armv7m_systick.c | 78
>> ++++++++++++++++++++++++++++++++++++
>>   3 files changed, 86 insertions(+)
>>   create mode 100644 drivers/clocksource/armv7m_systick.c
>>
>> diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
>> index 1c2506f..b82e58b 100644
>> --- a/drivers/clocksource/Kconfig
>> +++ b/drivers/clocksource/Kconfig
>> @@ -134,6 +134,13 @@ config CLKSRC_ARM_GLOBAL_TIMER_SCHED_CLOCK
>>         help
>>          Use ARM global timer clock source as sched_clock
>>
>> +config ARMV7M_SYSTICK
>> +       bool
>> +       select CLKSRC_OF if OF
>> +       select CLKSRC_MMIO
>> +       help
>> +         This options enables support for the ARMv7M system timer unit
>> +
>>   config ATMEL_PIT
>>         select CLKSRC_OF if OF
>>         def_bool SOC_AT91SAM9 || SOC_SAMA5
>> diff --git a/drivers/clocksource/Makefile b/drivers/clocksource/Makefile
>> index 752d5c7..1c9a643 100644
>> --- a/drivers/clocksource/Makefile
>> +++ b/drivers/clocksource/Makefile
>> @@ -44,6 +44,7 @@ obj-$(CONFIG_MTK_TIMER)               += mtk_timer.o
>>
>>   obj-$(CONFIG_ARM_ARCH_TIMER)          += arm_arch_timer.o
>>   obj-$(CONFIG_ARM_GLOBAL_TIMER)                += arm_global_timer.o
>> +obj-$(CONFIG_ARMV7M_SYSTICK)           += armv7m_systick.o
>>   obj-$(CONFIG_CLKSRC_METAG_GENERIC)    += metag_generic.o
>>   obj-$(CONFIG_ARCH_HAS_TICK_BROADCAST) += dummy_timer.o
>>   obj-$(CONFIG_ARCH_KEYSTONE)           += timer-keystone.o
>> diff --git a/drivers/clocksource/armv7m_systick.c
>> b/drivers/clocksource/armv7m_systick.c
>> new file mode 100644
>> index 0000000..23d8249
>> --- /dev/null
>> +++ b/drivers/clocksource/armv7m_systick.c
>> @@ -0,0 +1,78 @@
>> +/*
>> + * Copyright (C) Maxime Coquelin 2015
>> + * Author:  Maxime Coquelin <mcoquelin.stm32@xxxxxxxxx>
>> + * License terms:  GNU General Public License (GPL), version 2
>> + */
>> +
>> +#include <linux/kernel.h>
>> +#include <linux/clocksource.h>
>> +#include <linux/clockchips.h>
>> +#include <linux/of.h>
>> +#include <linux/of_address.h>
>> +#include <linux/clk.h>
>> +#include <linux/bitops.h>
>> +
>> +#define SYST_CSR       0x00
>> +#define SYST_RVR       0x04
>> +#define SYST_CVR       0x08
>> +#define SYST_CALIB     0x0c
>> +
>> +#define SYST_CSR_ENABLE BIT(0)
>> +
>> +#define SYSTICK_LOAD_RELOAD_MASK 0x00FFFFFF
>> +
>> +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?

>
>> +       }
>> +
>> +       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?

Regards,
Maxime

>
>> +               clk_disable_unprepare(clk);
>> +out_unmap:
>> +       iounmap(base);
>> +       WARN(ret, "ARM System timer register failed (%d)\n", ret);
>> +}
>> +
>> +CLOCKSOURCE_OF_DECLARE(arm_systick, "arm,armv7m-systick",
>> +                       system_timer_of_register);
>>
>
>
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