linux-kernel - Re: [PATCH] riscv: Add riscv_force_qspinlock for early

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Message-ID: <c6974bec-851e-4fdf-8af3-23cfdc9dd50f@ghiti.fr>
Date: Wed, 26 Feb 2025 10:27:57 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: Guo Ren <guoren@...nel.org>
Cc: conor@...nel.org, alexghiti@...osinc.com,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 paul.walmsley@...ive.com, palmer@...belt.com, bjorn@...osinc.com,
 leobras@...hat.com, peterz@...radead.org, parri.andrea@...il.com,
 longman@...hat.com, boqun.feng@...il.com, arnd@...db.de, will@...nel.org,
 corbet@....net, Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH] riscv: Add riscv_force_qspinlock for early_param

Hi Guo,

First sorry I did not follow up on this patch.

On 22/12/2024 05:03, Guo Ren wrote:
> On Mon, Dec 16, 2024 at 5:15 PM Alexandre Ghiti <alex@...ti.fr> wrote:
>> Hi Guo,
>>
>> On 14/12/2024 05:35, guoren@...nel.org wrote:
>>> From: Guo Ren <guoren@...ux.alibaba.com>
>>>
>>> When CONFIG_RISCV_COMBO_SPINLOCKS is enabled, permit qspinlock
>>> force enabled. See the Kconfig entry for RISCV_COMBO_SPINLOCKS.
>>>
>>> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
>>> Signed-off-by: Guo Ren <guoren@...nel.org>
>>> ---
>>>    Documentation/admin-guide/kernel-parameters.txt |  5 +++++
>>>    arch/riscv/kernel/setup.c                       | 13 ++++++++++++-
>>>    2 files changed, 17 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>> index 3872bc6ec49d..43d0df2922b2 100644
>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>> @@ -5887,6 +5887,11 @@
>>>                        [KNL] Disable ring 3 MONITOR/MWAIT feature on supported
>>>                        CPUs.
>>>
>>> +     riscv_force_qspinlock [RISCV, EARLY]
>>> +                     When CONFIG_RISCV_COMBO_SPINLOCKS is enabled, permit
>>> +                     qspinlock force enabled. See the Kconfig entry for
>>> +                     RISCV_COMBO_SPINLOCKS.
>>> +
>>>        riscv_isa_fallback [RISCV,EARLY]
>>>                        When CONFIG_RISCV_ISA_FALLBACK is not enabled, permit
>>>                        falling back to detecting extension support by parsing
>>> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
>>> index 45010e71df86..74b13bc64c9c 100644
>>> --- a/arch/riscv/kernel/setup.c
>>> +++ b/arch/riscv/kernel/setup.c
>>> @@ -247,6 +247,15 @@ static void __init parse_dtb(void)
>>>    #if defined(CONFIG_RISCV_COMBO_SPINLOCKS)
>>>    DEFINE_STATIC_KEY_TRUE(qspinlock_key);
>>>    EXPORT_SYMBOL(qspinlock_key);
>>> +
>>> +static bool force_qspinlock;
>>> +
>>> +static int __init riscv_force_qspinlock(char *p)
>>> +{
>>> +     force_qspinlock = true;
>>> +     return 0;
>>> +}
>>> +early_param("riscv_force_qspinlock", riscv_force_qspinlock);
>>>    #endif
>>>
>>>    static void __init riscv_spinlock_init(void)
>>> @@ -267,7 +276,9 @@ static void __init riscv_spinlock_init(void)
>>>                using_ext = "using Ziccrse";
>>>        }
>>>    #if defined(CONFIG_RISCV_COMBO_SPINLOCKS)
>>> -     else {
>>> +     else if (force_qspinlock) {
>>> +             using_ext = "force";
>>> +     } else {
>>>                static_branch_disable(&qspinlock_key);
>>>                pr_info("Ticket spinlock: enabled\n");
>>>                return;
>>
>> What's the use case for this early param? To me that implies that a
>> platform may have another extension which would allow the usage of
> We want to use it for sg2042 & th1520. No new extension was
> introduced, and some micro-architecture could give LR/SC
> implementation a forward progress guarantee more than the spec
> required.


But then those platforms support Ziccrse which is enough to use 
qspinlocks right?

Thanks,

Alex


>
>> qspinlock, so why not listing it in riscv_spinlock_init() instead?
> List all platforms in riscv_spinlock_init is noisy, maybe give a
> qspinlock param in cmdline, and they could put it in their boot args.
>
>> Thanks,
>>
>> Alex
>>
>