Re: [PATCH V4 08/23] RISC-V: ACPI: Cache and retrieve the RINTC structure

[Sunil V L]

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@xxxxxxxxxxxxxxxx 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@xxxxxxxxxxxxxxxx>
> > Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > ---
> >  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.

Thanks!
Sunil