Message-ID: <aad3213b-1dcd-4619-9719-fa4585012cc8@ghiti.fr>
Date: Tue, 2 Sep 2025 08:55:24 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: liu.xuemei1@....com.cn, paul.walmsley@...ive.com
Cc: palmer@...belt.com, aou@...s.berkeley.edu, spersvold@...il.com,
 sudeep.holla@....com, mikisabate@...il.com, robh@...nel.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: cacheinfo: init cache levels via fetch_cache_info
 when SMP disabled

Hi Jessica,

On 8/14/25 10:16, Alexandre Ghiti wrote:
> Hi Jessica,
>
> On 8/14/25 03:29, liu.xuemei1@....com.cn wrote:
>>
>> Hi Alex,
>>
>>
>> >> Hi Jessica,
>>
>> >>
>>
>> >> On 8/1/25 03:32, liu.xuemei1@....com.cn wrote:
>>
>> >>>
>>
>> >>> On 7/31/25 21:29, alex@...ti.fr wrote:
>>
>> >>>
>>
>> >>> > > From: Jessica Liu <liu.xuemei1@....com.cn>
>>
>> >>>
>>
>> >>> > >
>>
>> >>>
>>
>> >>> > > As described in commit 1845d381f280 ("riscv: cacheinfo: Add back
>>
>> >>>
>>
>> >>> > > init_cache_level() function"), when CONFIG_SMP is undefined, the
>>
>> >>> cache
>>
>> >>>
>>
>> >>> > > hierarchy detection needs to be performed through the
>>
>> >>> init_cache_level(),
>>
>> >>>
>>
>> >>> > > whereas when CONFIG_SMP is defined, this detection is handled
>>
>> >>> during the
>>
>> >>>
>>
>> >>> > > init_cpu_topology() process.
>>
>> >>>
>>
>> >>> > >
>>
>> >>>
>>
>> >>> > > Furthermore, while commit 66381d36771e ("RISC-V: Select ACPI 
>> PPTT
>>
>> >>> drivers")
>>
>> >>>
>>
>> >>> > > enables cache information retrieval through the ACPI PPTT 
>> table, the
>>
>> >>>
>>
>> >>> > > init_of_cache_level() called within init_cache_level() cannot
>>
>> >>> support cache
>>
>> >>>
>>
>> >>> > > hierarchy detection through ACPI PPTT. Therefore, when 
>> CONFIG_SMP is
>>
>> >>>
>>
>> >>> > > undefined, we directly invoke the fetch_cache_info function to
>>
>> >>> initialize
>>
>> >>>
>>
>> >>> > > the cache levels.
>>
>> >>>
>>
>> >>> > >
>>
>> >>>
>>
>> >>> > > Signed-off-by: Jessica Liu <liu.xuemei1@....com.cn>
>>
>> >>>
>>
>> >>> > > ---
>>
>> >>>
>>
>> >>> > >   arch/riscv/kernel/cacheinfo.c | 6 +++++-
>>
>> >>>
>>
>> >>> > >   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> >>>
>>
>> >>> > >
>>
>> >>>
>>
>> >>> > > diff --git a/arch/riscv/kernel/cacheinfo.c
>>
>> >>> b/arch/riscv/kernel/cacheinfo.c
>>
>> >>>
>>
>> >>> > > index 26b085dbdd07..f81ca963d177 100644
>>
>> >>>
>>
>> >>> > > --- a/arch/riscv/kernel/cacheinfo.c
>>
>> >>>
>>
>> >>> > > +++ b/arch/riscv/kernel/cacheinfo.c
>>
>> >>>
>>
>> >>> > > @@ -73,7 +73,11 @@ static void ci_leaf_init(struct cacheinfo
>>
>> >>> *this_leaf,
>>
>> >>>
>>
>> >>> > >
>>
>> >>>
>>
>> >>> > >   int init_cache_level(unsigned int cpu)
>>
>> >>>
>>
>> >>> > >   {
>>
>> >>>
>>
>> >>> > > -    return init_of_cache_level(cpu);
>>
>> >>>
>>
>> >>> > > +#ifdef CONFIG_SMP
>>
>> >>>
>>
>> >>> > > +    return 0;
>>
>> >>>
>>
>> >>> > > +#endif
>>
>> >>>
>>
>> >>> > > +
>>
>> >>>
>>
>> >>> > > +    return fetch_cache_info(cpu);
>>
>> >>>
>>
>> >>> > >   }
>>
>> >>>
>>
>> >>> > >
>>
>> >>>
>>
>> >>> > >   int populate_cache_leaves(unsigned int cpu)
>>
>> >>>
>>
>> >>> >
>>
>> >>>
>>
>> >>> >
>>
>> >>>
>>
>> >>> > Is the current behaviour wrong or just redundant? If wrong, 
>> I'll add a
>>
>> >>>
>>
>> >>> > Fixes tag to backport, otherwise I won't.
>>
>> >>>
>>
>> >>> >
>>
>> >>>
>>
>> >>> > Thanks,
>>
>> >>>
>>
>> >>> >
>>
>> >>>
>>
>> >>> > Alex
>>
>> >>>
>>
>> >>>
>>
>> >>> Hi Alex,
>>
>> >>>
>>
>> >>>
>>
>> >>> The current behavior is actually wrong when using ACPI on 
>> !CONFIG_SMP
>>
>> >>>
>>
>> >>> systems. The original init_of_cache_level() cannot detect cache
>>
>> >>> hierarchy
>>
>> >>>
>>
>> >>> through ACPI PPTT table, which means cache information would be 
>> missing
>>
>> >>>
>>
>> >>> in this configuration.
>>
>> >>>
>>
>> >>>
>>
>> >>> The patch fixes this by directly calling fetch_cache_info() when
>>
>> >>>
>>
>> >>> CONFIG_SMP is undefined, which properly handles both DT and ACPI 
>> cases..
>>
>> >>>
>>
>> >>>
>>
>> >>> So yes, it would be appropriate to add a Fixes tag. The commit being
>>
>> >>>
>>
>> >>> fixed is 1845d381f280 ("riscv: cacheinfo: Add back 
>> init_cache_level()
>>
>> >>> function").
>>
>> >>>
>>
>> >>>
>>
>> >>> Please let me know if you need any additional information.
>>
>> >>>
>>
>> >>
>>
>> >> I'm about to send my first PR for 6.17 so I'll delay merging this one
>>
>> >> for the first rc.
>>
>> >
>>
>> >
>>
>> >So I took the time this morning to look into this, and I don't really
>>
>> >like the different treatment for smp, can't we just move
>>
>> >init_cpu_topology() call to setup_arch() (or else) for both !smp and 
>> smp?
>>
>> >
>>
>> >Thanks,
>>
>> >
>>
>> >Alex
>>
>>
>> Thank you for your feedback and suggestion. I understand your desire
>>
>> to have a unified approach for both SMP and !SMP. However, after
>>
>> careful consideration, I still believe that handling them separately
>>
>> is the more appropriate solution.
>>
>>
>> The current method of obtaining cache information in
>>
>> `init_cpu_topology()` is specific to RISC-V and ARM64. If we move
>>
>> `init_cpu_topology()` to cover both SMP and !SMP, it may require
>>
>> modifying the generic boot sequence. This could inadvertently affect
>>
>> other architectures that do not rely on `init_cpu_topology()` for
>>
>> cache initialization, leading to potential regressions and maintenance
>>
>> issues.
>>
>>
>> The `setup_arch()` function is called early in the boot process,
>>
>> and at this stage, the ACPI subsystem has not been fully initialized.
>>
>> Specifically, the ACPI tables (including PPTT) are not yet parsed.
>>
>> Therefore, if we call `init_cpu_topology()` from `setup_arch()`, it
>>
>> would not be able to retrieve cache information from the ACPI PPTT 
>> table.
>>
>>
>> I hope this clarifies my train of thought. I'm open to further 
>> discussion and
>>
>> alternative suggestions that can address the issue properly.
>>
>
> To me it does not make sense to retrieve the cache info at 2 different 
> points in time if the system is smp or not. I still think we should 
> find a common place where init_cpu_topology() can be called for both 
> smp and up, setup_arch() could not be the right place for the reasons 
> you gave, but we just need to find the right one :)
>
> Thanks for working on this,


I don't mean to pressure you, I know it's the end of summer and people 
are still on vacations or just back from vacation.

I just wanted to know if you had time to look into what I asked above?

Thanks,

Alex


>
> Alex
>
>
>>
>> Best regards,
>>
>> Jessica
>>
>>
>>
>>
>>
>>
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

Powered by blists - more mailing lists