Timer enablement with ACPI

Many functions in Linux kernel need time management, such as periodic scheduler, delay program and timer. The hardware timer provides the clock source. The frequency of the clock source can be set. After setting, the timing interrupt will be generated periodically. The system uses the timing interrupt to time. The frequency generated periodically by interrupts is the system frequency, also known as the tick rate.

HW requirement

For a Arm Linux system, it require HW support generic timer for system scheduler, and external timer for CPU idle requirement.

System counter generate event (Arm system ready recommend it to 1GHz) to CPU private timer (called as local timer), or to external timer (can be acted as global timer)

CPU private time MAY be stop when CPU enter idle state, at this time system should switch to broadcast time like global timer in below picture.

For NXP i.MX8MP platform, it integrates 6 GPT external timers, which can be registered as system global timer. And, the system counter in i.MX8MP also support compare function that means it can be registered as one clock event. Since system counter is in always-on domain, so the clock event can be used as system global timer. Once cpu enter idle, system can switch cpu private timer to the system counter.

ACPI requirement

For ACPI on arm64, tables also fall into the following categories:

Required: DSDT, FADT, GTDT, MADT, MCFG, RSDP, SPCR, XSDT

Generic Timer Description Table (GTDT) provides OSPM with information about a system’s Generic Timers configuration. The Generic Timer (GT) is a standard timer interface implemented on ARM processor-based systems. The GT hardware specification can be found at Links to ACPI-Related Documents (http://uefi.org/acpi ) under the heading ARM Architecture.
The GTDT provides OSPM with information about a system's GT interrupt configurations, for both perprocessor timers, and platform (memory-mapped) timers.

The GT specification defines the following per-processor timers:

Secure EL1 timer,
Non-Secure EL1 timer,
EL2 timer,
Virtual EL1 timer,
Virtual EL2 timer,

and defines the following memory-mapped Platform timers:

GT Block,
Server Base System Architecture (SBSA) Generic Watchdog.

GTDT table structure

ACPI table defined in UEFI:

typedef struct {
  UINT32  Signature;
  UINT32  Length;
  UINT8   Revision;
  UINT8   Checksum;
  UINT8   OemId[6];
  UINT64  OemTableId;
  UINT32  OemRevision;
  UINT32  CreatorId;
  UINT32  CreatorRevision;
} EFI_ACPI_DESCRIPTION_HEADER;

///
/// Generic Timer Description Table definition.
///
typedef struct {
  EFI_ACPI_DESCRIPTION_HEADER    Header;
  UINT64                         PhysicalAddress;
  UINT32                         GlobalFlags;
  UINT32                         SecurePL1TimerGSIV;
  UINT32                         SecurePL1TimerFlags;
  UINT32                         NonSecurePL1TimerGSIV;
  UINT32                         NonSecurePL1TimerFlags;
  UINT32                         VirtualTimerGSIV;
  UINT32                         VirtualTimerFlags;
  UINT32                         NonSecurePL2TimerGSIV;
  UINT32                         NonSecurePL2TimerFlags;
} EFI_ACPI_5_0_GENERIC_TIMER_DESCRIPTION_TABLE;

SW Status

Generic timer work well on i.MX8MP.

Code for ACPI

GTDT table for i.MX8MP:

# ARM Architectural Timer Interrupt(GIC PPI) numbers
  gArmTokenSpaceGuid.PcdArmArchTimerSecIntrNum|29|UINT32|0x00000035
  gArmTokenSpaceGuid.PcdArmArchTimerIntrNum|30|UINT32|0x00000036
  gArmTokenSpaceGuid.PcdArmArchTimerHypIntrNum|26|UINT32|0x00000040
  gArmTokenSpaceGuid.PcdArmArchTimerVirtIntrNum|27|UINT32|0x00000041
  

#define GTDT_GLOBAL_FLAGS_MAPPED      EFI_ACPI_5_0_GTDT_GLOBAL_FLAG_MEMORY_MAPPED_BLOCK_PRESENT
#define GTDT_GLOBAL_FLAGS_NOT_MAPPED  0
#define GTDT_GLOBAL_FLAGS_EDGE        EFI_ACPI_5_0_GTDT_GLOBAL_FLAG_INTERRUPT_MODE
#define GTDT_GLOBAL_FLAGS_LEVEL       0

// Note: We could have a build flag that switches between memory mapped/non-memory mapped timer
#ifdef SYSTEM_TIMER_BASE_ADDRESS
  #define GTDT_GLOBAL_FLAGS             (GTDT_GLOBAL_FLAGS_MAPPED | GTDT_GLOBAL_FLAGS_LEVEL)
#else
  #define GTDT_GLOBAL_FLAGS             (GTDT_GLOBAL_FLAGS_NOT_MAPPED | GTDT_GLOBAL_FLAGS_LEVEL)
  #define SYSTEM_TIMER_BASE_ADDRESS     0xFFFFFFFFFFFFFFFF
#endif

#define GTDT_TIMER_EDGE_TRIGGERED   EFI_ACPI_5_0_GTDT_TIMER_FLAG_TIMER_INTERRUPT_MODE
#define GTDT_TIMER_LEVEL_TRIGGERED  0
#define GTDT_TIMER_ACTIVE_LOW       EFI_ACPI_5_0_GTDT_TIMER_FLAG_TIMER_INTERRUPT_POLARITY
#define GTDT_TIMER_ACTIVE_HIGH      0

#define GTDT_GTIMER_FLAGS           (GTDT_TIMER_ACTIVE_LOW | GTDT_TIMER_LEVEL_TRIGGERED)

  EFI_ACPI_5_0_GENERIC_TIMER_DESCRIPTION_TABLE Gtdt = {
    ARM_ACPI_HEADER(
      EFI_ACPI_5_0_GENERIC_TIMER_DESCRIPTION_TABLE_SIGNATURE,
      EFI_ACPI_5_0_GENERIC_TIMER_DESCRIPTION_TABLE,
      EFI_ACPI_5_0_GENERIC_TIMER_DESCRIPTION_TABLE_REVISION
    ),
    SYSTEM_TIMER_BASE_ADDRESS,                            // UINT64  PhysicalAddress
    GTDT_GLOBAL_FLAGS,                                            // UINT32  GlobalFlags
    FixedPcdGet32 (PcdArmArchTimerSecIntrNum),    // UINT32  SecurePL1TimerGSIV
    GTDT_GTIMER_FLAGS,                                             // UINT32  SecurePL1TimerFlags
    FixedPcdGet32 (PcdArmArchTimerIntrNum),          // UINT32  NonSecurePL1TimerGSIV
    GTDT_GTIMER_FLAGS,                                             // UINT32  NonSecurePL1TimerFlags
    FixedPcdGet32 (PcdArmArchTimerVirtIntrNum),    // UINT32  VirtualTimerGSIV
    GTDT_GTIMER_FLAGS,                                             // UINT32  VirtualTimerFlags
    FixedPcdGet32 (PcdArmArchTimerHypIntrNum),   // UINT32  NonSecurePL2TimerGSIV
    GTDT_GTIMER_FLAGS                                              // UINT32  NonSecurePL2TimerFlags
  };