[PATCH v4 0/4] x86/of: Add support for interrupt mode config for x86 OF systems

[Rahul Tanwar <rtanwar@maxlinear.com>]

Hi All,

This patch series mainly adds a boot time interrupt delivery mode
configuration option for OF based x86 platforms. Presently,
boot time interrupt delivery mode is hardcoded to legacy PIC mode
with no option to configure it to virtual wire mode. This patch
series aims to extend it by introducing a new optional boolean
property for lapic devicetree node which can be used to configure
it to virtual wire mode where applicable. Please find below detailed
rationale behind it in case you want more details about it.

Rationale:

References [1], [2] & [6]

For SMP systems, Intel defines three (logically four) interrupt modes
during boot/init time while BIOS/bootloader boots & switches to linux
kernel.

  1. PIC mode - Legacy 8259 PIC interrupt controller.
  2. Virtual wire mode via Local APIC - uses local APIC as virtual wire
  3. Virtual wire mode via I/O APIC - uses I/O APIC as virtual wire
  4. Symmetric I/O mode - final one used by linux for SMP systems. 

BIOS/bootloaders are supposed to boot in either #1 or #2 or #3 and then
switch to #4 in linux for SMP systems.

For our platform, we use #2.

Detection of which interrupt mode the system is booting in is made by using
below global variable in apic.c

int pic_mode __ro_after_init; 

Here pic_mode = 1 means #1 (PIC mode) above.
And pic_mode = 0 means #2 or #3 (basically virtual wire mode via apic).

And apic.c while doing setup_local_APIC() uses below code [3]:

        value = apic_read(APIC_LVT0) & APIC_LVT_MASKED;
        if (!cpu && (pic_mode || !value || skip_ioapic_setup)) {
                value = APIC_DM_EXTINT;
                apic_printk(APIC_VERBOSE, "enabled ExtINT on CPU#%d\n", cpu);
        } else {
                value = APIC_DM_EXTINT | APIC_LVT_MASKED;
                apic_printk(APIC_VERBOSE, "masked ExtINT on CPU#%d\n", cpu);
        }
        apic_write(APIC_LVT0, value);

What i understand from above is that if at this point of time, as long as
it is cpu0 & pic_mode=1, it will set delivery mode to ExtINT (causes the
processor to respond to the interrupt as if the interrupt originated in an
externally connected (8259A-compatible) interrupt controller) and enables/
unmask the interrupts. This causes kernel boot crash for platforms which
does not support 8259 compatible external PIC.

pic_mode is presently set/populated/initialized at only two places:
 1. In  mpparse.c [4]
 2. In devicetree.c [7]

For #1 MPPARSE Kconfig definition is as below:

	config X86_MPPARSE
        	bool "Enable MPS table" if ACPI
        	default y
        	depends on X86_LOCAL_APIC
        	help
          	For old smp systems that do not have proper acpi support. Newer systems
          	(esp with 64bit cpus) with acpi support, MADT and DSDT will override it

As seen above, if ACPI is not enabled, then mpparse by default is always
enabled. Presently, there is no way to disable MPPARSE (if ACPI is not
enabled). This to me appears to be a bug which needs fixing. As per
theory, MPPARSE was to support MPS spec [1] as a temporary solution to
support SMP systems until a final ACPI standard was added. But now if ACPI
is not enabled, it will rely on MPPARSE driver to read MP floating pointer
structure's IMCRP Bit 7 of MP feature info byte 2 [5] to figure out if it
supports PIC mode or virtual wire mode and initialize pic_mode variable
accordingly. If ACPI is enabled, the ACPI code overrides it by using the
MADT table spec'ed in ACPI spec [2]. 

For #2 devicetree.c presently hardcodes pic_mode = 1 (PIC Mode). There is
no support to configure virtual wire mode via devicetree path for OF based
systems.

Now we have a platform which is OF based & does not use legacy 8259 PIC
interrupt controller. Non ACPI compliant as well as non MPPARSE compliant.

For such platforms, it appears to me that hardcoding pic_mode = 1 (PIC Mode)
and giving no other choice to choose virtual wire mode is a lacking feature.

Just like mpparse relies on IMCRP bit 7 of MP feature info byte2 [5] to
select pic_mode to PIC mode or virtual wire mode. arch/x86/kernel/devicetree.c
should also provide some similar configurability to choose interrupt
delivery mode & not hardcode it to PIC mode.

This patch is to add above mentioned interrupt mode configurability in x86/of
controlled via a new optional bool property.

Please let me know if you find any mistake in above understanding or if you
have a alternative better suggestion to solve it or if you find anything odd
here in our platform/system. TIA.

The patch is baselined on below git tree (linux-v6.1.0-rc6):
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

[1] https://pdos.csail.mit.edu/6.828/2008/readings/ia32/MPspec.pdf
[2] https://uefi.org/sites/default/files/resources/ACPI_6_3_final_Jan30.pdf
[3] https://elixir.bootlin.com/linux/v6.1-rc5/source/arch/x86/kernel/apic/apic.c#L1691
[4] https://elixir.bootlin.com/linux/v6.1-rc5/source/arch/x86/kernel/mpparse.c#L517
[5] https://www.manualslib.com/manual/77733/Intel-Multiprocessor.html?page=40#manual
[6] https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdm.html
[7] https://elixir.bootlin.com/linux/v6.1-rc5/source/arch/x86/kernel/devicetree.c#L170

v4:
- Address review concerns from Andy Shevchenko
  * Update maintainers in binding files.
  * Place URL in YAML schema properly as reference.
  * Remove some unnecessary comments from YAML description.
  * Remove fixes tag & not treat it as a bug. Treat it as new feature addition instead.
  * Use proper prefixes for bindings file (dt-bindings: x86: ioapic:)
  * Add Reviewed-by tag from Andy for patch 3/4.

v3:
- Address review concerns from Andy Shevchenko
  * Reshuffle patch series changes to make it more logical.
  * Patch 1 just converts existing intel,ce4100-ioapic.txt into
    YAML schema and separates out ioapic & lapic.
  * Patch 2 adds new optional property for lapic.
  * Patch 3 replaces older printk(KERN_LVL) to newer pr_lvl()
  * Patch 4 adds code changes in devicetree.c to support newly
    added property.
- Fix 'make DT_CHECKER_FLAGS=-m dt_binding_check' errors reported
  by Rob Herring's bot.

v2:
- Address review concern from Andy - rename property name to make
  it a bit more positive & self explanatory.
- Found that the bindings document for these HW's (APIC) are a bit
  off/obsolete and still in text format. Created new YAML schemas
  one for each - lapic & ioapic. Updated these schemas with latest
  info and add in new optional property details in the updated
  schema for lapic. Delete/let go of the text binding doc.
- CC devicetree@vger.kernel.org as these changes appear to be
  mainly targeted for devicetree maintainers review & approval.
- Increase CCed list to include all possible people who touched
  and were involved this part of code/feature addition.

v1:
- Initial draft


Rahul Tanwar (4):
  dt-bindings: x86: apic: Convert Intel's APIC bindings to YAML schema
  dt-bindings: x86: apic: Introduce new optional bool property for lapic
  x86/of: Replace printk(KERN_LVL) with pr_lvl()
  x86/of: Add support for boot time interrupt delivery mode
    configuration

 .../intel,ce4100-ioapic.txt                   | 26 --------
 .../intel,ce4100-ioapic.yaml                  | 62 ++++++++++++++++++
 .../intel,ce4100-lapic.yaml                   | 63 +++++++++++++++++++
 arch/x86/kernel/devicetree.c                  | 13 +++-
 4 files changed, 135 insertions(+), 29 deletions(-)
 delete mode 100644 Documentation/devicetree/bindings/interrupt-controller/intel,ce4100-ioapic.txt
 create mode 100644 Documentation/devicetree/bindings/interrupt-controller/intel,ce4100-ioapic.yaml
 create mode 100644 Documentation/devicetree/bindings/interrupt-controller/intel,ce4100-lapic.yaml

-- 
2.17.1