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Date:   Thu, 27 Apr 2023 12:25:42 +0200
From:   Andrew Jones <ajones@ventanamicro.com>
To:     Sunil V L <sunilvl@ventanamicro.com>
Cc:     Palmer Dabbelt <palmer@dabbelt.com>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
        linux-acpi@vger.kernel.org, linux-crypto@vger.kernel.org,
        platform-driver-x86@vger.kernel.org, llvm@lists.linux.dev,
        corbet@lwn.net, Paul Walmsley <paul.walmsley@sifive.com>,
        aou@eecs.berkeley.edu, lenb@kernel.org, daniel.lezcano@linaro.org,
        tglx@linutronix.de, qianweili@huawei.com, wangzhou1@hisilicon.com,
        herbert@gondor.apana.org.au, Marc Zyngier <maz@kernel.org>,
        luzmaximilian@gmail.com, hdegoede@redhat.com, markgross@kernel.org,
        nathan@kernel.org, ndesaulniers@google.com, trix@redhat.com,
        rafael@kernel.org, davem@davemloft.net, rafael.j.wysocki@intel.com
Subject: Re: [PATCH V4 08/23] RISC-V: ACPI: Cache and retrieve the RINTC
 structure
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References: <20230404182037.863533-9-sunilvl@ventanamicro.com>
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 <ZEo+6rwM+c6DvlMM@sunil-laptop>
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On Thu, Apr 27, 2023 at 02:52:50PM +0530, Sunil V L wrote:
> Hi Palmer,
> 
> On Wed, Apr 26, 2023 at 11:45:00AM -0700, Palmer Dabbelt wrote:
> > On Tue, 04 Apr 2023 11:20:22 PDT (-0700), sunilvl@ventanamicro.com wrote:
> > > RINTC structures in the MADT provide mapping between the hartid
> > > and the CPU. This is required many times even at run time like
> > > cpuinfo. So, instead of parsing the ACPI table every time, cache
> > > the RINTC structures and provide a function to get the correct
> > > RINTC structure for a given cpu.
> > > 
> > > Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
> > > Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > ---
> > >  arch/riscv/include/asm/acpi.h |  2 ++
> > >  arch/riscv/kernel/acpi.c      | 60 +++++++++++++++++++++++++++++++++++
> > >  2 files changed, 62 insertions(+)
> > > 
> > > diff --git a/arch/riscv/include/asm/acpi.h b/arch/riscv/include/asm/acpi.h
> > > index 9be52b6ffae1..1606dce8992e 100644
> > > --- a/arch/riscv/include/asm/acpi.h
> > > +++ b/arch/riscv/include/asm/acpi.h
> > > @@ -59,6 +59,8 @@ static inline bool acpi_has_cpu_in_madt(void)
> > > 
> > >  static inline void arch_fix_phys_package_id(int num, u32 slot) { }
> > > 
> > > +struct acpi_madt_rintc *acpi_cpu_get_madt_rintc(int cpu);
> > > +u32 get_acpi_id_for_cpu(int cpu);
> > >  #endif /* CONFIG_ACPI */
> > > 
> > >  #endif /*_ASM_ACPI_H*/
> > > diff --git a/arch/riscv/kernel/acpi.c b/arch/riscv/kernel/acpi.c
> > > index 81d448c41714..40ab55309c70 100644
> > > --- a/arch/riscv/kernel/acpi.c
> > > +++ b/arch/riscv/kernel/acpi.c
> > > @@ -24,6 +24,66 @@ EXPORT_SYMBOL(acpi_disabled);
> > >  int acpi_pci_disabled = 1;	/* skip ACPI PCI scan and IRQ initialization */
> > >  EXPORT_SYMBOL(acpi_pci_disabled);
> > > 
> > > +static struct acpi_madt_rintc cpu_madt_rintc[NR_CPUS];
> > > +
> > > +static int acpi_parse_madt_rintc(union acpi_subtable_headers *header, const unsigned long end)
> > > +{
> > > +	struct acpi_madt_rintc *rintc = (struct acpi_madt_rintc *)header;
> > > +	int cpuid;
> > > +
> > > +	if (!(rintc->flags & ACPI_MADT_ENABLED))
> > > +		return 0;
> > > +
> > > +	cpuid = riscv_hartid_to_cpuid(rintc->hart_id);
> > 
> > Unless I'm missing something, this races with CPUs coming online.  Maybe
> > that's a rare enough case we don't care, but I think we'd also just have
> > simpler logic if we fixed it...
> > 
> This depend only on cpuid_to_hartid_map filled up. I wish I could
> initialize this RINTC mapping in setup_smp() itself like ARM64. But in
> RISC-V, this file smpboot.c gets built only when CONFIG_SMP is enabled.
> Hence, we need to initialize this array outside of setup_smp().
> 
> I can update the code to initialize this from setup_arch() immediately
> after setup_smp() if ACPI is enabled. That should avoid the global
> variable check also. Let me know if you prefer this.
> 
> > > +	/*
> > > +	 * When CONFIG_SMP is disabled, mapping won't be created for
> > > +	 * all cpus.
> > > +	 * CPUs more than NR_CPUS, will be ignored.
> > > +	 */
> > > +	if (cpuid >= 0 && cpuid < NR_CPUS)
> > > +		cpu_madt_rintc[cpuid] = *rintc;
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int acpi_init_rintc_array(void)
> > > +{
> > > +	if (acpi_table_parse_madt(ACPI_MADT_TYPE_RINTC, acpi_parse_madt_rintc, 0) > 0)
> > > +		return 0;
> > > +
> > > +	return -ENODEV;
> > > +}
> > > +
> > > +/*
> > > + * Instead of parsing (and freeing) the ACPI table, cache
> > > + * the RINTC structures since they are frequently used
> > > + * like in  cpuinfo.
> > > + */
> > > +struct acpi_madt_rintc *acpi_cpu_get_madt_rintc(int cpu)
> > > +{
> > > +	static bool rintc_init_done;
> > 
> > ... basically just get rid of this global variable, and instead have a
> > 
> >    if (!&cpu_madt_rintc[cpu])
> >        ... parse ...
> >    return &cpu_madt_rintc[cpu];
> > 
> > that'd probably let us get rid of a handful of these helpers too, as now
> > it's just a call to the parsing bits.
> > 
> I am afraid this (!&cpu_madt_rintc[cpu]) check won't work since we are
> not caching the RINTC pointers but actual contents itself. So, the
> address is always valid. However, as per Drew's earlier feedback I am
> going to reduce one helper. I am planning to send the next version of
> this patch once 6.4 rc1 is available since the ACPICA patches are merged
> now.
> 
> > > +
> > > +	if (!rintc_init_done) {
> > > +		if (acpi_init_rintc_array()) {
> > > +			pr_err("No valid RINTC entries exist\n");
> > > +			return NULL;
> > > +		}
> > > +
> > > +		rintc_init_done = true;
> > > +	}
> > > +
> > > +	return &cpu_madt_rintc[cpu];
> > > +}
> > > +
> > > +u32 get_acpi_id_for_cpu(int cpu)
> > > +{
> > > +	struct acpi_madt_rintc *rintc = acpi_cpu_get_madt_rintc(cpu);
> > > +
> > > +	BUG_ON(!rintc);
> > 
> > We should have some better error reporting here.  It looks like all the
> > callerss of get_acpi_id_for_cpu() are tolerant of a nonsense ID being
> > returned, so maybe we just pr_warn() something users can understand and then
> > return -1 or something?
> > 
> 
> RINTC is mandatory for ACPI systems. Also, all 32bit values are valid
> for UID. So, there is no bogus value we can return. 
> 
> Actually, I just realized this check is redundant. It will never be NULL
> since it is a static array. So, we can just get rid of the BUG.

It can be NULL on the first call of acpi_cpu_get_madt_rintc(), which is
a good time to BUG if there's isn't an RINTC.

Thanks,
drew