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RISC-V ACLINT

Introduction

  1. 功能

    原文

    This RISC-V ACLINT specification defines a set of memory mapped devices which provide inter-processor interrupts (IPI) and timer functionalities for each HART on a multi-HART RISC-V platform.

    ACLINT 是一組 memory mapped devices，用於在 multi-hart 的 RISC-V 平台上提供

    • inter-processor interrupts (IPI)
    • 定時器功能 (Timer functionalities)

  2. CLINT 的局限性

    原文

    The SiFive Core-Local Interruptor (CLINT) device has been widely adopted in the RISC-V world to provide machine-level IPI and timer functionalities.

    Unfortunately, the SiFive CLINT has a unified register map for both IPI and timer functionalities and it does not provide supervisor-level IPI functionality.

    • CLINT 在 RISC-V 平台上廣泛用於提供 Machine-level 的 IPI 和定時器功能
    • 局限性
      • unified register map
        • CLINT 將 IPI 和定時器功能的暫存器放在一個統一的地址空間中
      • 缺乏 supervisor-level IPI

  3. ACLINT 的改進

    原文

    The RISC-V ACLINT specification takes a more modular approach by defining separate memory mapped devices for IPI and timer functionalities.

    This modularity allows RISC-V platforms to omit some of the RISC-V ACLINT devices for when the platform has an alternate mechanism.

    In addition to modularity, the RISC-V ACLINT specification also defines a dedicated memory mapped device for supervisor-level IPIs.

    • modularity
      • ACLINT 將 IPI 和定時器功能分別定義為獨立的 memory mapped device
      • 如果平台有替代機制，可以選擇性地省略部分 ACLINT 設備
    • 另外 ACLINT 定義了專門的 memory mapped device 來支持 supervisor-level IPI，填補了 CLINT 的功能空缺

  4. ACLINT Devices

    Name Privilege Level Functionality
    MTIMER Machine Fixed-frequency counter and timer events
    MSWI Machine Inter-processor (or software) interrupts
    SSWI Supervisor Inter-processor (or software) interrupts

  5. 與 SiFive CLINT 的兼容性

    原文

    The RISC-V ACLINT specification is defined to be backward compatible with the SiFive CLINT specification.

    The register definitions and register offsets of the MTIMER and MSWI devices are compatible with the timer and IPI registers defined by the SiFive CLINT specification.

    • ACLINT 的 MTIMER 和 MSWI 設備的暫存器定義和偏移量與 SiFive CLINT 中的定時器和 IPI 暫存器保持一致

  6. CLINT 與 ACLINT 的邏輯關係

    SiFive CLINT 設備可以被邏輯地視為兩個 ACLINT 設備的組合

    SiFive CLINT Offset Range ACLINT Device Functionality
    0x0000_0000 - 0x0000_3fff MSWI Machine-level inter-processor (or software) interrupts
    0x0000_4000 - 0x0000_bfff MTIMER Machine-level fixed-frequency counter and timer events
    • MSWI：對應 CLINT 地址範圍 0x0000_0000 - 0x0000_3fff，提供 Machine-level IPI
    • MTIMER：對應 CLINT 地址範圍 0x0000_4000 - 0x0000_bfff，提供 Machine-level 定時器功能

Machine-level Timer Device (MTIMER)

Introduction

原文

The MTIMER device provides machine-level timer functionality for a set of HARTs on a RISC-V platform.

It has a single fixed-frequency monotonic time counter (MTIME) register and a time compare register (MTIMECMP) for each HART connected to the MTIMER device.

A MTIMER device not connected to any HART should only have a MTIME register and no MTIMECMP registers.

  • MTIMER 為一組 HART 提供 Machine-level 的定時器功能，用於支持計時和產生定時器中斷
  • MTIME Register
    • 每個 HART 有一個對應的 MTIME Register，其是一個單調遞增的固定頻率時間計數器，適用於所有連接到 MTIMER 的 HART
  • MTIMECMP Register
    • 每個 HART 有一個對應的 MTIMECMP Register，作為對應的時間比較暫存器
    • 當 MTIME 的值達到或超過 MTIMECMP 的值時，觸發 Machine-level 的定時器中斷
  • 如果 MTIMER 沒有連接到任何 HART
    • 它應該只包含 MTIME 暫存器
    • 不需要包含任何 MTIMECMP 暫存器，因為沒有 HART 需要時間比較功能
原文

On a RISC-V platform with multiple MTIMER devices:

  • Each MTIMER device provides machine-level timer functionality for a different
    (or disjoint) set of HARTs.

    A MTIMER device assigns a HART index starting from zero to each HART associated with it.

    The HART index assigned to a HART by the MTIMER device may or may not have any relationship with the unique HART identifier (hart ID) that the RISC-V Privileged Architecture assigns to the HART.

  • Two or more MTIMER devices can share the same physical MTIME register while having their own separate MTIMECMP registers.

  • The MTIMECMP registers of a MTIMER device must only compare against the MTIME register of the same MTIMER device for generating machine-level timer interrupt.

The maximum number of HARTs supported by a single MTIMER device is 4095 which is equivalent to the maximum number of MTIMECMP registers.

  • 每個 MTIMER 設備只負責一組不同（或不相交）的 HART 的 machine-level 定時器功能
  • HART Index
    • MTIMER 將從 0 開始分配 index 給所連接的 HART
    • index 與 RISC-V 特權架構分配的 HART ID 不一定有關
  • MTIME 暫存器共享
    • 多個 MTIMER 設備可以共用一個物理上的 MTIME 暫存器。
    • 但 MTIMER 設備仍擁有獨立的 MTIMECMP 暫存器，確保每個 HART 的比較和中斷處理是分開的
  • MTIMECMP 的比較規則
    • MTIMECMP 暫存器只能與同一 MTIMER 設備的 MTIME 暫存器進行比較
    • 這種設計確保中斷由 MTIMER 設備內部獨立管理
  • 每個 MTIMER 設備最多支持 4095 個 HART
    • 這與最多支持 4095 個 MTIMECMP 暫存器相對應

Register Map

原文

A MTIMER device has two separate base addresses: one for the MTIME register and another for the MTIMECMP registers.

These separate base addresses of a single MTIMER device allows multiple MTIMER devices to share the same physical MTIME register.

  • MTIMER 設備有兩個 base addresses
    • MTIME Register Base Address：提供 Machine-level 的時間計數功能
    • MTIMECMP Registers Base Address：為每個 HART 提供獨立的時間比較功能
    • 用來使不同的 MTIMER 設備可以共享一個物理上的 MTIME 暫存器，但擁有獨立的 MTIMECMP 暫存器

ACLINT MTIMER Time Register Map：

Offset Width Attr Name Description
0x0000_0000 8B RW MTIME Machine-level time counter

ACLINT MTIMER Compare Register Map：

Offset Width Attr Name Description
0x0000_0000 8B RW MTIMECMP0 HART index 0 machine-level time compare
0x0000_0008 8B RW MTIMECMP1 HART index 1 machine-level time compare
… … … … …
0x0000_7FF0 8B RW MTIMECMP4094 HART index 4094 machine-level time compare

MTIME Register (Offset: 0x00000000)

原文

The MTIME register is a 64-bit read-write register that contains the number of cycles counted based on a fixed reference frequency.

On MTIMER device reset, the MTIME register is cleared to zero.

  • MTIME 為 64-bit Read-Write 暫存器
  • 用於存儲基於固定參考頻率（fixed reference frequency）的計數器值
  • MTIMER reset 時，暫存器的值清為 0
  • 提供單調遞增的時間基準，供 MTIMECMP 暫存器進行比較
  • 用於觸發 Machine-level timer 中斷

MTIMECMP Registers (Offsets: 0x00000000 - 0x00007FF0)

原文

The MTIMECMP registers are per-HART 64-bit read-write registers.

It contains the MTIME register value at which machine-level timer interrupt is to be triggered for the corresponding HART.

  • 每個 HART 分配一個對應的 64-bit Read-Write MTIMECMP 暫存器
  • 存儲一個 Target Value，當 MTIME 的值達到或超過此值時，對應的 HART 會觸發 Machine-level timer 中斷
原文

The machine-level timer interrupt of a HART is pending whenever MTIME is greater than or equal to the value in the corresponding MTIMECMP register.

The machine-level timer interrupt of a HART is cleared whenever MTIME is less than the value of the corresponding MTIMECMP register.

The machine-level timer interrupt is reflected in the MTIP bit of the mip CSR.

On MTIMER device reset, the MTIMECMP registers are in unknown state.

  • 中斷觸發條件：
    • 當 MTIME >= MTIMECMP 時，對應 HART 的 Machine-level timer 中斷處於 pending 狀態。
  • 中斷清除條件：
    • 當 MTIME < MTIMECMP 時，對應 HART 的 Machine-level timer 中斷被清除
  • 中斷狀態會反映在每個 HART 的 mip CSR 的 MTIP bit
  • MTIMER 設備重置後，所有 MTIMECMP 暫存器處於未知狀態

Synchronizing Multiple MTIME Registers

原文

A RISC-V platform can have multiple HARTs grouped into hierarchical topology groups (such as clusters, nodes, or sockets) where each topology group has its own MTIMER device.

Further, such RISC-V platforms can also allow clock-gating or powering off for a topology group (including the MTIMER device) at runtime.

  • 在多 HART 的 RISC-V 平台上，HART 可以按照 clusters、nodes 或 sockets 劃分為拓撲組，每個拓撲組可以擁有獨立的 MTIMER 設備
  • 平台可以允許對整個拓撲組（包括 MTIMER 設備）進行 clock-gating 或關機的操作
原文

On a RISC-V platform with multiple MTIMER devices residing on the same die, each device must satisfy the RISC-V architectural requirement that all the MTIME registers with respect to each other, and all the per-HART time CSRs with respect to each other, are synchronized to within one MTIME tick period.

For example, if the MTIME tick period is 10ns, then the MTIME registers, and their associated time CSRs, should respectively be synchronized to within 10ns of each other.

  • 同一晶片內
    • 所有 MTIME Registers 和每個 HART 的 time CSRs 必須在一個 MTIME tick 週期內同步
    • 舉例來說，若 MTIME tick period 為 10ns，則同步偏差應限制在 10ns 內
原文

On a RISC-V platform with multiple MTIMER devices on different die, the MTIME registers (and their associated time CSRs) on different die may be synchronized to only within a specified interval of each other that is larger than the MTIME tick period.

A platform may define a maximum allowed interval.

  • 不同晶片中
    • 各晶片的 MTIME Registers 和 time CSRs 可以允許更大的同步偏差
    • 平台可定義允許的最大同步間隔
原文

To satisfy the preceding MTIME synchronization requirements:

  • All MTIME registers should have the same input clock so as to avoid runtime drift between separate MTIME registers (and their associated time CSRs)

  • Upon system reset, the hardware must initialize and synchronize all MTIME registers to zero

  • When a MTIMER device is stopped and started again due to, say, power management actions, the software should re-synchronize this MTIME register with all other MTIME registers

  • 所有 MTIME Registers 應使用相同的輸入時鐘，避免運行時的時鐘有誤差
  • 系統重置時，硬體需初始化並同步所有 MTIME Registers 為零
  • 當 MTIMER 因功耗管理等原因停止並重新啟動時，軟體需重新將該 MTIME Register 與其他 MTIME Registers 同步
原文

When software updates one, multiple, or all MTIME registers, it must maintain the preceding synchronization requirements (through measuring and then taking into account the differing latencies of performing reads or writes to the different MTIME registers).

  • 當軟體更新 MTIME Registers 時，需測量並補償不同暫存器的讀寫延遲以滿足前面的同步要求
原文

As an example, the below RISC-V 64-bit assembly sequence can be used by software to synchronize a MTIME register with reference to another MTIME register.

NOTE: On some RISC-V platforms, the MTIME synchronization sequence (i.e. the aclint_mtime_sync() function above) will need to be repeated few times until delta between target MTIME register and reference MTIME register is zero (or very close to zero).

以下為一範例 asm code，將目標 MTIME Register 與參考 MTIME Register 進行同步：

/*
 * unsigned long aclint_mtime_sync(unsigned long target_mtime_address,
 *                                 unsigned long reference_mtime_address)
 */
        .globl aclint_mtime_sync
aclint_mtime_sync:
        /* Read target MTIME register in T0 register */
        ld        t0, (a0)
        fence     i, i

        /* Read reference MTIME register in T1 register */
        ld        t1, (a1)
        fence     i, i

        /* Read target MTIME register in T2 register */
        ld        t2, (a0)
        fence     i, i

        /*
         * Compute target MTIME adjustment in T3 register
         * T3 = T1 - ((T0 + T2) / 2)
         */
        srli      t0, t0, 1
        srli      t2, t2, 1
        add       t3, t0, t2
        sub       t3, t1, t3

        /* Update target MTIME register */
        ld        t4, (a0)
        add       t4, t4, t3
        sd        t4, (a0)

        /* Return MTIME adjustment value */
        add       a0, t3, zero

        ret

某些平台可能需要多次重複同步過程，直到目標與參考 MTIME Register 之間的偏差接近零

Machine-level Software Interrupt Device (MSWI)

Introduction

原文

The MSWI device provides machine-level IPI functionality for a set of HARTs on a RISC-V platform.

It has an IPI register (MSIP) for each HART connected to the MSWI device.

On a RISC-V platform with multiple MSWI devices, each MSWI device provides machine-level IPI functionality for a different (or disjoint) set of HARTs.

A MSWI device assigns a HART index starting from zero to each HART associated with it.

The HART index assigned to a HART by the MSWI device may or may not have any relationship with the unique HART identifier (hart ID) that the RISC-V Privileged Architecture assigns to the HART.

The maximum number of HARTs supported by a single MSWI device is 4095 which is equivalent to the maximum number of MSIP registers.

  • MSWI 設備為一組 HART 提供 Machine-level IPI 的功能
  • 對於連接到 MSWI 設備的每個 HART，它都有一個對應的 IPI 暫存器（MSIP）
  • 一個平台可以有多個 MSWI 設備，每個設備負責不同（或不相交）的一組 HART
  • HART Index
    • 每個 MSWI 設備從 index 0 開始為其連接的 HART 分配一個 HART index
    • 該 index 用於標識 HART，但不一定與 RISC-V 特權架構分配的 HART ID 有關
  • 每個 MSWI 設備最多支持 4095 個 HART，這與最多支持 4095 個 MSIP Registers 相對應

Register Map

ACLINT MSWI Device Register Map：

Offset Width Attr Name Description
0x0000_0000 4B RW MSIP0 HART index 0 machine-level IPI register
0x0000_0004 4B RW MSIP1 HART index 1 machine-level IPI register
… … … … …
0x0000_3FFC 4B RESERVED Reserved for future use.
  • addresse offset
    • 第 0 個 HART 的 MSIP 位於 0x0000_0000
    • 第 1 個 HART 的 MSIP 位於 0x0000_0004
    • 依此類推，每個暫存器占用 4 字節（32 位）
  • 地址範圍 0x0000_3FFC 為保留區域，用於未來擴展

MSIP Registers (Offsets: 0x00000000 - 0x00003FF8)

原文

Each MSIP register is a 32-bit wide WARL register where the upper 31 bits are wired to zero.

The least significant bit is reflected in MSIP of the mip CSR.

A machine-level software interrupt for a HART is pending or cleared by writing 1 or 0 respectively to the corresponding MSIP register.

On MSWI device reset, each MSIP register is cleared to zero.

  • 每個 MSIP 暫存器都是一個 32 位元寬的 WARL 暫存器
  • 高 31 位硬接線為 0
  • 最低有效位（LSB）反映 mip CSR 的 MSIP bit
  • 寫入 1 可設置中斷掛起
  • 寫入 0 可清除中斷
  • MSWI 設備重設時，每個 MSIP 暫存器都清零

Supervisor-level Software Interrupt Device (SSWI)

Introduction

原文

The SSWI device provides supervisor-level IPI functionality for a set of HARTs on a RISC-V platform.

It provides a register to set an IPI (SETSSIP) for each HART connected to the SSWI device.

  • SSWI 設備為一組 HART 提供 Supervisor-level IPI 功能
  • 對於連接到 SSWI 設備的每個 HART，它都有一個對應的 IPI 暫存器（SETSSIP）
原文

On a RISC-V platform with multiple SSWI devices, each SSWI device provides supervisor-level IPI functionality for a different (or disjoint) set of HARTs.

A SSWI device assigns a HART index starting from zero to each HART associated with it.

The HART index assigned to a HART by the SSWI device may or may not have any relationship with the unique HART identifier (hart ID) that the RISC-V Privileged Architecture assigns to the HART.

The maximum number of HARTs supported by a single SSWI device is 4095 which is equivalent to the maximum number of SETSSIP registers.

  • 在具有多個 SSWI 設備的 RISC-V 平台上，每個 SSWI 設備為不同（或不相交）的一組 HART 提供 Supervisor-level IPI 功能
  • HART Index
    • 每個 SSWI 設備從 index 0 開始為其連接的 HART 分配一個 HART index
    • 該 index 用於標識 HART，但不一定與 RISC-V 特權架構分配的 HART ID 有關
  • 每個 SSWI 設備最多支持 4095 個 HART，對應到 4095 個 SETSSIP Registers

Register Map

ACLINT SSWI Device Register Map：

Offset Width Attr Name Description
0x0000_0000 4B RW SETSSIP0 HART index 0 set supervisor-level IPI register
0x0000_0004 4B RW SETSSIP1 HART index 1 set supervisor-level IPI register
… … … … …
0x0000_3FFC 4B RESERVED Reserved for future use.
  • addresse offset
    • 第 0 個 HART 的 MSIP 位於 0x0000_0000
    • 第 1 個 HART 的 MSIP 位於 0x0000_0004
    • 依此類推，每個暫存器占用 4 字節（32 位）
  • 地址範圍 0x0000_3FFC 為保留區域，用於未來擴展

SETSSIP Registers (Offsets: 0x00000000 - 0x00003FF8)

原文

Each SETSSIP register is a 32-bit wide WARL register where the upper 31 bits are wired to zero.

The least significant bit of a SETSSIP register always reads 0. Writing 0 to the least significant bit of a SETSSIP register has no effect whereas writing 1 to the least significant bit sends an edge-sensitive interrupt signal to the corresponding HART causing the HART to set SSIP in the mip CSR.

Writes to a SETSSIP register are guaranteed to be reflected in SSIP of the corresponding HART but not necessarily immediately.

  • 每個 SETSSIP 暫存器都是一個 32 位元寬的 WARL 暫存器
  • 高 31 位硬接線為 0
  • 讀取 SETSSIP 暫存器的最低有效位元始終返回 0
  • 將 0 寫入 SETSSIP 暫存器的最低有效位元不會產生任何影響
  • 將 1 寫入最低有效位元會向對應的 HART 發送 edge-sensitive 中斷訊號，導致 HART 在 mip CSR 中設定 SSIP
  • 對 SETSSIP 暫存器的寫入操作保證會反映在對應 HART 的 SSIP 中，但不一定會立即反映
原文

NOTE: The RISC-V Privileged Architecture defines SSIP in mip and sip CSRs as a writeable bit so the M-mode or S-mode software can directly clear SSIP.

  • RISC-V 特權架構中定義 SSIP mip 和 sip CSR 作為可寫位，因此 M mode 或 S mode 中軟體可以直接清除 SSIP
  • 黃色部分不確定是拼錯還是有意為之
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