Re: [PATCH] clocksource: dw_apb_timer_of: support timer-based delay

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

On 11/02/2015 03:51 AM, Jisheng Zhang wrote:
> Dear Daniel,
>
> On Fri, 30 Oct 2015 13:37:01 +0100
> Daniel Lezcano wrote:
>
>> On 10/30/2015 12:09 PM, Jisheng Zhang wrote:
>>> Dear Daniel,
>>>
>>> On Fri, 30 Oct 2015 11:44:46 +0100
>>> Daniel Lezcano <daniel.lezcano@....> wrote:
>>>
>>>> On 10/30/2015 09:27 AM, Jisheng Zhang wrote:
>>>>> Implement an ARM delay timer to be used for udelay(). This allows us to
>>>>> skip the delay loop calibration at boot on Marvell BG2, BG2Q, BG2CD
>>>>> platforms. And after this patch, udelay() will be unaffected by CPU
>>>>> frequency changes.
>>>>>
>>>>> Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
>>>>> ---
>>>>>     drivers/clocksource/Kconfig           | 10 ++++++++++
>>>>>     drivers/clocksource/dw_apb_timer_of.c | 16 ++++++++++++++++
>>>>>     2 files changed, 26 insertions(+)
>>>>>
>>>>> diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
>>>>> index a7726db..7b081805 100644
>>>>> --- a/drivers/clocksource/Kconfig
>>>>> +++ b/drivers/clocksource/Kconfig
>>>>> @@ -29,6 +29,16 @@ config DW_APB_TIMER_OF
>>>>>     	select DW_APB_TIMER
>>>>>     	select CLKSRC_OF
>>>>>
>>>>> +config DW_APB_TIMER_BASED_DELAY
>>>>> +	bool "DW APB timer based delay"
>>>>> +	depends on ARM && DW_APB_TIMER_OF
>>>>> +	default n
>>>>> +	help
>>>>> +	  This option enables support for using the DW APB timer to
>>>>> +	  implement timer-based delay. It is useful for skiping the
>>>>> +	  delay loop calibration at boot on some platforms. And the
>>>>> +	  udelay() will be unaffected by CPU frequency changes.
>>>>> +
>>>>
>>>> Why do you want it to be optional ?
>>>>
>>>
>>> Because in some platforms which has arm arch timer, this dw apb timer
>>> delay isn't needed, the arch timer is better. So we want it be optional
>>> so that the platforms which need this feature select it manually when config
>>> the kernel.
>>
>> Correct me if I am wrong. If you have the arch timer, you don't need the
>
> Yes, I don't need the dw apb timer if we have arch timer,
>
>> dw apb timer at all, no ? So the selection would be arch arm timer *or*
>> dw_apb_timer ? not arch_arm_timer for delay and dw_apb_timer for
>> clockevents, right ?
>
> Yes, if we have arch timer, I prefer to use it for clockevent and delay.
>
> Could you please provide suggestion how to handle this case?

If I follow the logic of arch_arm_timer is better than dw_apb timer.

1. The arch_arm_timer is present

  => dw_apb timer is not used at all

  CONFIG_ARM_ARCH_TIMER=y
  # CONFIG_DW_APB_TIMER is not set

2. The arch_arm_timer is *not* present

  => dw_apb_timer is used with delay code

  # CONFIG_ARM_ARCH_TIMER is not set
  CONFIG_DW_APB_TIMER=y

In both cases, DW_APB_TIMER_BASED_DELAY is not needed.


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