linux-kernel - Re: Loongson (and other $ARCHs?) idle VS timer enqueue

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Message-ID: <b882eb61-d783-985e-7687-386f9e03235d@xen0n.name>
Date:   Wed, 26 Apr 2023 10:10:41 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     maobibo <maobibo@...ngson.cn>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Huacai Chen <chenhuacai@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Loongson (and other $ARCHs?) idle VS timer enqueue

On 2023/4/26 08:46, maobibo wrote:
> 
> 
> 在 2023/4/25 21:28, WANG Xuerui 写道:
>> On 2023/4/25 21:25, maobibo wrote:
>>>
>>>
>>> 在 2023/4/25 19:49, Peter Zijlstra 写道:
>>
>> <snip>
>>
>>>>
>>>> diff --git a/arch/loongarch/kernel/genex.S b/arch/loongarch/kernel/genex.S
>>>> index 44ff1ff64260..3c8a6bab98fe 100644
>>>> --- a/arch/loongarch/kernel/genex.S
>>>> +++ b/arch/loongarch/kernel/genex.S
>>>> @@ -18,27 +18,31 @@
>>>>          .align    5
>>>>    SYM_FUNC_START(__arch_cpu_idle)
>>>> -    /* start of rollback region */
>>>> -    LONG_L    t0, tp, TI_FLAGS
>>>> -    nop
>>>> -    andi    t0, t0, _TIF_NEED_RESCHED
>>>> -    bnez    t0, 1f
>>>> -    nop
>>>> -    nop
>>>> -    nop
>>>> +    /* start of idle interrupt region */
>>>> +    move    t0, CSR_CRMD_IE
>>> addi.d  t0, zero, CSR_CRMD_IE can be used here, move is used for reg to reg
>>
>> Or better: li.d t0, CSR_CRMD_IE (prefer pseudo-instruction over concrete ones whenever it helps readability). We don't need to support ancient in-house toolchains without support for even li. ;-)
>    
> I am not familiar with compiler:(, how many actual instructions does
> pseudo-instr li.d takes? It will be ok if it uses only one intr, else
> there will be problem.

It's just `ori $t0, $zero, 4` no matter which of li.w or li.d is used. 
It only matters when the immediate to load is bigger. Given CSR_CRMD_IE 
is just 4 (1<<2) you can definitely say `li.w` if you want to be extra 
cautious and it won't hurt. ;-)

-- 
WANG "xen0n" Xuerui

Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/