Re: [PATCH v7 2/3] clocksource: Add JH7110 timer driver

[Xingyu Wu <xingyu.wu@starfivetech.com>]

On 2023/11/11 2:02, Daniel Lezcano wrote:
> 
> Hi Samuel,
> 
> On 10/11/2023 18:40, Samuel Holland wrote:
>> On 2023-11-08 11:51 PM, Xingyu Wu wrote:
>>> On 2023/11/8 17:10, Daniel Lezcano wrote:
>>>> On 08/11/2023 04:45, Xingyu Wu wrote:
>>>>> On 2023/11/2 22:29, Daniel Lezcano wrote:
>>>>
>>>> [ ... ]
>>>>
>>>>> Thanks. The riscv-timer has a clocksource with a higher rating but a
>>>>> clockevent with lower rating[1] than jh7110-timer. I tested the
>>>>> jh7110-timer as clockevent and flagged as one shot, which could do some
>>>>> of the works instead of riscv-timer. And the current_clockevent changed
>>>>> to jh7110-timer.
>>>>>
>>>>> Because the jh7110-timer works as clocksource with lower rating and only
>>>>> will be used as global timer at CPU idle time. Is it necessary to be
>>>>> registered as clocksource? If not, should it just be registered as
>>>>> clockevent?
>>>>
>>>> Yes, you can register the clockevent without the clocksource.
>>>>
>>>> You mentioned the JH7110 has a better rating than the CPU architected
>>>> timers. The rating is there to "choose" the best timer, so it is up to the
>>>> author of the driver check against which timers it compares on the
>>>> platform.
>>>>
>>>> Usually, CPU timers are the best.
>>>>
>>>> It is surprising the timer-riscv has a so low rating. You may double check
>>>> if jh7110 is really better. If it is the case, then implementing a
>>>> clockevent per cpu would make more sense, otherwise one clockevent as a
>>>> global timer is enough.
>>
>> The timer-riscv clockevent has a low rating because it requires a call to
>> firmware to set the timer, as well as a trap to firmware to handle the
>> interrupt, which both add overhead. Implementations which support the Sstc
>> extension[1] do not require firmware assistance to implement the clockevent, so
>> in that case we register the clockevent with a higher rating.
>>
>> [1]: https://github.com/riscv/riscv-time-compare
> 
> Thanks for the pointer and the clarification.
> 
>>>> Unused clocksource, clockevents should be stopped in case the firmware let
>>>> them in a undetermined state.
>>>
>>> The interrupts of jh7110-timer each channel are global interrupts like
>>> SPI(Shared Peripheral Interrupt) not PPI (Private Peripheral Interrupt). They
>>> are up to PLIC to select which core to respond to. So it is hard to implement
>>> a clockevent per cpu core. I tested this with request_percpu_irq() and it
>>> failed.
>>
>> You cannot use request_percpu_irq(), but the driver should be able to set the
>> affinity of each IRQ to a separate CPU.
> 
> Absolutely. And given the bad rating of the local timers, it may be worth to implement this driver in a per CPU (affinity set) basis.
> 
> At the first glance, the arm_global_timer can be used as an example.
> 
> Note in this case, you may want to double check what does with an idle state with a local timer stop flag and this timer which is always on.
> 
> 
> 

Hi Daniel and Samuel,

Thanks for your pointers. I will check it. If it works, I will send the new version of this patch.

Best regards,
Xingyu Wu

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