Documentation: LoongArch: Add basic documentations

Add some basic documentation for LoongArch. LoongArch is a new RISC ISA,
which is a bit like MIPS or RISC-V. LoongArch includes a reduced 32-bit
version (LA32R), a standard 32-bit version (LA32S) and a 64-bit version
(LA64).

Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Co-developed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

1 files changed, 387 insertions, 0 deletions

diff --git a/Documentation/loongarch/introduction.rst b/Documentation/loongarch/introduction.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+Introduction to LoongArch
+=========================
+
+LoongArch is a new RISC ISA, which is a bit like MIPS or RISC-V. There are
+currently 3 variants: a reduced 32-bit version (LA32R), a standard 32-bit
+version (LA32S) and a 64-bit version (LA64). There are 4 privilege levels
+(PLVs) defined in LoongArch: PLV0~PLV3, from high to low. Kernel runs at PLV0
+while applications run at PLV3. This document introduces the registers, basic
+instruction set, virtual memory and some other topics of LoongArch.
+
+Registers
+=========
+
+LoongArch registers include general purpose registers (GPRs), floating point
+registers (FPRs), vector registers (VRs) and control status registers (CSRs)
+used in privileged mode (PLV0).
+
+GPRs
+----
+
+LoongArch has 32 GPRs ( ``$r0`` ~ ``$r31`` ); each one is 32-bit wide in LA32
+and 64-bit wide in LA64. ``$r0`` is hard-wired to zero, and the other registers
+are not architecturally special. (Except ``$r1``, which is hard-wired as the
+link register of the BL instruction.)
+
+The kernel uses a variant of the LoongArch register convention, as described in
+the LoongArch ELF psABI spec, in :ref:`References <loongarch-references>`:
+
+================= =============== =================== ============
+Name              Alias           Usage               Preserved
+                                                      across calls
+================= =============== =================== ============
+``$r0``           ``$zero``       Constant zero       Unused
+``$r1``           ``$ra``         Return address      No
+``$r2``           ``$tp``         TLS/Thread pointer  Unused
+``$r3``           ``$sp``         Stack pointer       Yes
+``$r4``-``$r11``  ``$a0``-``$a7`` Argument registers  No
+``$r4``-``$r5``   ``$v0``-``$v1`` Return value        No
+``$r12``-``$r20`` ``$t0``-``$t8`` Temp registers      No
+``$r21``          ``$u0``         Percpu base address Unused
+``$r22``          ``$fp``         Frame pointer       Yes
+``$r23``-``$r31`` ``$s0``-``$s8`` Static registers    Yes
+================= =============== =================== ============
+
+Note: The register ``$r21`` is reserved in the ELF psABI, but used by the Linux
+kernel for storing the percpu base address. It normally has no ABI name, but is
+called ``$u0`` in the kernel. You may also see ``$v0`` or ``$v1`` in some old code,
+however they are deprecated aliases of ``$a0`` and ``$a1`` respectively.
+
+FPRs
+----
+
+LoongArch has 32 FPRs ( ``$f0`` ~ ``$f31`` ) when FPU is present. Each one is
+64-bit wide on the LA64 cores.
+
+The floating-point register convention is the same as described in the
+LoongArch ELF psABI spec:
+
+================= ================== =================== ============
+Name              Alias              Usage               Preserved
+                                                         across calls
+================= ================== =================== ============
+``$f0``-``$f7``   ``$fa0``-``$fa7``  Argument registers  No
+``$f0``-``$f1``   ``$fv0``-``$fv1``  Return value        No
+``$f8``-``$f23``  ``$ft0``-``$ft15`` Temp registers      No
+``$f24``-``$f31`` ``$fs0``-``$fs7``  Static registers    Yes
+================= ================== =================== ============
+
+Note: You may see ``$fv0`` or ``$fv1`` in some old code, however they are deprecated
+aliases of ``$fa0`` and ``$fa1`` respectively.
+
+VRs
+----
+
+There are currently 2 vector extensions to LoongArch:
+
+- LSX (Loongson SIMD eXtension) with 128-bit vectors,
+- LASX (Loongson Advanced SIMD eXtension) with 256-bit vectors.
+
+LSX brings ``$v0`` ~ ``$v31`` while LASX brings ``$x0`` ~ ``$x31`` as the vector
+registers.
+
+The VRs overlap with FPRs: for example, on a core implementing LSX and LASX,
+the lower 128 bits of ``$x0`` is shared with ``$v0``, and the lower 64 bits of
+``$v0`` is shared with ``$f0``; same with all other VRs.
+
+CSRs
+----
+
+CSRs can only be accessed from privileged mode (PLV0):
+
+================= ===================================== ==============
+Address           Full Name                             Abbrev Name
+================= ===================================== ==============
+0x0               Current Mode Information              CRMD
+0x1               Pre-exception Mode Information        PRMD
+0x2               Extension Unit Enable                 EUEN
+0x3               Miscellaneous Control                 MISC
+0x4               Exception Configuration               ECFG
+0x5               Exception Status                      ESTAT
+0x6               Exception Return Address              ERA
+0x7               Bad (Faulting) Virtual Address        BADV
+0x8               Bad (Faulting) Instruction Word       BADI
+0xC               Exception Entrypoint Address          EENTRY
+0x10              TLB Index                             TLBIDX
+0x11              TLB Entry High-order Bits             TLBEHI
+0x12              TLB Entry Low-order Bits 0            TLBELO0
+0x13              TLB Entry Low-order Bits 1            TLBELO1
+0x18              Address Space Identifier              ASID
+0x19              Page Global Directory Address for     PGDL
+                  Lower-half Address Space
+0x1A              Page Global Directory Address for     PGDH
+                  Higher-half Address Space
+0x1B              Page Global Directory Address         PGD
+0x1C              Page Walk Control for Lower-          PWCL
+                  half Address Space
+0x1D              Page Walk Control for Higher-         PWCH
+                  half Address Space
+0x1E              STLB Page Size                        STLBPS
+0x1F              Reduced Virtual Address Configuration RVACFG
+0x20              CPU Identifier                        CPUID
+0x21              Privileged Resource Configuration 1   PRCFG1
+0x22              Privileged Resource Configuration 2   PRCFG2
+0x23              Privileged Resource Configuration 3   PRCFG3
+0x30+n (0≤n≤15)   Saved Data register                   SAVEn
+0x40              Timer Identifier                      TID
+0x41              Timer Configuration                   TCFG
+0x42              Timer Value                           TVAL
+0x43              Compensation of Timer Count           CNTC
+0x44              Timer Interrupt Clearing              TICLR
+0x60              LLBit Control                         LLBCTL
+0x80              Implementation-specific Control 1     IMPCTL1
+0x81              Implementation-specific Control 2     IMPCTL2
+0x88              TLB Refill Exception Entrypoint       TLBRENTRY
+                  Address
+0x89              TLB Refill Exception BAD (Faulting)   TLBRBADV
+                  Virtual Address
+0x8A              TLB Refill Exception Return Address   TLBRERA
+0x8B              TLB Refill Exception Saved Data       TLBRSAVE
+                  Register
+0x8C              TLB Refill Exception Entry Low-order  TLBRELO0
+                  Bits 0
+0x8D              TLB Refill Exception Entry Low-order  TLBRELO1
+                  Bits 1
+0x8E              TLB Refill Exception Entry High-order TLBEHI
+                  Bits
+0x8F              TLB Refill Exception Pre-exception    TLBRPRMD
+                  Mode Information
+0x90              Machine Error Control                 MERRCTL
+0x91              Machine Error Information 1           MERRINFO1
+0x92              Machine Error Information 2           MERRINFO2
+0x93              Machine Error Exception Entrypoint    MERRENTRY
+                  Address
+0x94              Machine Error Exception Return        MERRERA
+                  Address
+0x95              Machine Error Exception Saved Data    MERRSAVE
+                  Register
+0x98              Cache TAGs                            CTAG
+0x180+n (0≤n≤3)   Direct Mapping Configuration Window n DMWn
+0x200+2n (0≤n≤31) Performance Monitor Configuration n   PMCFGn
+0x201+2n (0≤n≤31) Performance Monitor Overall Counter n PMCNTn
+0x300             Memory Load/Store WatchPoint          MWPC
+                  Overall Control
+0x301             Memory Load/Store WatchPoint          MWPS
+                  Overall Status
+0x310+8n (0≤n≤7)  Memory Load/Store WatchPoint n        MWPnCFG1
+                  Configuration 1
+0x311+8n (0≤n≤7)  Memory Load/Store WatchPoint n        MWPnCFG2
+                  Configuration 2
+0x312+8n (0≤n≤7)  Memory Load/Store WatchPoint n        MWPnCFG3
+                  Configuration 3
+0x313+8n (0≤n≤7)  Memory Load/Store WatchPoint n        MWPnCFG4
+                  Configuration 4
+0x380             Instruction Fetch WatchPoint          FWPC
+                  Overall Control
+0x381             Instruction Fetch WatchPoint          FWPS
+                  Overall Status
+0x390+8n (0≤n≤7)  Instruction Fetch WatchPoint n        FWPnCFG1
+                  Configuration 1
+0x391+8n (0≤n≤7)  Instruction Fetch WatchPoint n        FWPnCFG2
+                  Configuration 2
+0x392+8n (0≤n≤7)  Instruction Fetch WatchPoint n        FWPnCFG3
+                  Configuration 3
+0x393+8n (0≤n≤7)  Instruction Fetch WatchPoint n        FWPnCFG4
+                  Configuration 4
+0x500             Debug Register                        DBG
+0x501             Debug Exception Return Address        DERA
+0x502             Debug Exception Saved Data Register   DSAVE
+================= ===================================== ==============
+
+ERA, TLBRERA, MERRERA and DERA are sometimes also known as EPC, TLBREPC, MERREPC
+and DEPC respectively.
+
+Basic Instruction Set
+=====================
+
+Instruction formats
+-------------------
+
+LoongArch instructions are 32 bits wide, belonging to 9 basic instruction
+formats (and variants of them):
+
+=========== ==========================
+Format name Composition
+=========== ==========================
+2R          Opcode + Rj + Rd
+3R          Opcode + Rk + Rj + Rd
+4R          Opcode + Ra + Rk + Rj + Rd
+2RI8        Opcode + I8 + Rj + Rd
+2RI12       Opcode + I12 + Rj + Rd
+2RI14       Opcode + I14 + Rj + Rd
+2RI16       Opcode + I16 + Rj + Rd
+1RI21       Opcode + I21L + Rj + I21H
+I26         Opcode + I26L + I26H
+=========== ==========================
+
+Rd is the destination register operand, while Rj, Rk and Ra ("a" stands for
+"additional") are the source register operands. I8/I12/I16/I21/I26 are
+immediate operands of respective width. The longer I21 and I26 are stored
+in separate higher and lower parts in the instruction word, denoted by the "L"
+and "H" suffixes.
+
+List of Instructions
+--------------------
+
+For brevity, only instruction names (mnemonics) are listed here; please see the
+:ref:`References <loongarch-references>` for details.
+
+
+1. Arithmetic Instructions::
+
+    ADD.W SUB.W ADDI.W ADD.D SUB.D ADDI.D
+    SLT SLTU SLTI SLTUI
+    AND OR NOR XOR ANDN ORN ANDI ORI XORI
+    MUL.W MULH.W MULH.WU DIV.W DIV.WU MOD.W MOD.WU
+    MUL.D MULH.D MULH.DU DIV.D DIV.DU MOD.D MOD.DU
+    PCADDI PCADDU12I PCADDU18I
+    LU12I.W LU32I.D LU52I.D ADDU16I.D
+
+2. Bit-shift Instructions::
+
+    SLL.W SRL.W SRA.W ROTR.W SLLI.W SRLI.W SRAI.W ROTRI.W
+    SLL.D SRL.D SRA.D ROTR.D SLLI.D SRLI.D SRAI.D ROTRI.D
+
+3. Bit-manipulation Instructions::
+
+    EXT.W.B EXT.W.H CLO.W CLO.D SLZ.W CLZ.D CTO.W CTO.D CTZ.W CTZ.D
+    BYTEPICK.W BYTEPICK.D BSTRINS.W BSTRINS.D BSTRPICK.W BSTRPICK.D
+    REVB.2H REVB.4H REVB.2W REVB.D REVH.2W REVH.D BITREV.4B BITREV.8B BITREV.W BITREV.D
+    MASKEQZ MASKNEZ
+
+4. Branch Instructions::
+
+    BEQ BNE BLT BGE BLTU BGEU BEQZ BNEZ B BL JIRL
+
+5. Load/Store Instructions::
+
+    LD.B LD.BU LD.H LD.HU LD.W LD.WU LD.D ST.B ST.H ST.W ST.D
+    LDX.B LDX.BU LDX.H LDX.HU LDX.W LDX.WU LDX.D STX.B STX.H STX.W STX.D
+    LDPTR.W LDPTR.D STPTR.W STPTR.D
+    PRELD PRELDX
+
+6. Atomic Operation Instructions::
+
+    LL.W SC.W LL.D SC.D
+    AMSWAP.W AMSWAP.D AMADD.W AMADD.D AMAND.W AMAND.D AMOR.W AMOR.D AMXOR.W AMXOR.D
+    AMMAX.W AMMAX.D AMMIN.W AMMIN.D
+
+7. Barrier Instructions::
+
+    IBAR DBAR
+
+8. Special Instructions::
+
+    SYSCALL BREAK CPUCFG NOP IDLE ERTN(ERET) DBCL(DBGCALL) RDTIMEL.W RDTIMEH.W RDTIME.D
+    ASRTLE.D ASRTGT.D
+
+9. Privileged Instructions::
+
+    CSRRD CSRWR CSRXCHG
+    IOCSRRD.B IOCSRRD.H IOCSRRD.W IOCSRRD.D IOCSRWR.B IOCSRWR.H IOCSRWR.W IOCSRWR.D
+    CACOP TLBP(TLBSRCH) TLBRD TLBWR TLBFILL TLBCLR TLBFLUSH INVTLB LDDIR LDPTE
+
+Virtual Memory
+==============
+
+LoongArch supports direct-mapped virtual memory and page-mapped virtual memory.
+
+Direct-mapped virtual memory is configured by CSR.DMWn (n=0~3), it has a simple
+relationship between virtual address (VA) and physical address (PA)::
+
+ VA = PA + FixedOffset
+
+Page-mapped virtual memory has arbitrary relationship between VA and PA, which
+is recorded in TLB and page tables. LoongArch's TLB includes a fully-associative
+MTLB (Multiple Page Size TLB) and set-associative STLB (Single Page Size TLB).
+
+By default, the whole virtual address space of LA32 is configured like this:
+
+============ =========================== =============================
+Name         Address Range               Attributes
+============ =========================== =============================
+``UVRANGE``  ``0x00000000 - 0x7FFFFFFF`` Page-mapped, Cached, PLV0~3
+``KPRANGE0`` ``0x80000000 - 0x9FFFFFFF`` Direct-mapped, Uncached, PLV0
+``KPRANGE1`` ``0xA0000000 - 0xBFFFFFFF`` Direct-mapped, Cached, PLV0
+``KVRANGE``  ``0xC0000000 - 0xFFFFFFFF`` Page-mapped, Cached, PLV0
+============ =========================== =============================
+
+User mode (PLV3) can only access UVRANGE. For direct-mapped KPRANGE0 and
+KPRANGE1, PA is equal to VA with bit30~31 cleared. For example, the uncached
+direct-mapped VA of 0x00001000 is 0x80001000, and the cached direct-mapped
+VA of 0x00001000 is 0xA0001000.
+
+By default, the whole virtual address space of LA64 is configured like this:
+
+============ ====================== ======================================
+Name         Address Range          Attributes
+============ ====================== ======================================
+``XUVRANGE`` ``0x0000000000000000 - Page-mapped, Cached, PLV0~3
+             0x3FFFFFFFFFFFFFFF``
+``XSPRANGE`` ``0x4000000000000000 - Direct-mapped, Cached / Uncached, PLV0
+             0x7FFFFFFFFFFFFFFF``
+``XKPRANGE`` ``0x8000000000000000 - Direct-mapped, Cached / Uncached, PLV0
+             0xBFFFFFFFFFFFFFFF``
+``XKVRANGE`` ``0xC000000000000000 - Page-mapped, Cached, PLV0
+             0xFFFFFFFFFFFFFFFF``
+============ ====================== ======================================
+
+User mode (PLV3) can only access XUVRANGE. For direct-mapped XSPRANGE and
+XKPRANGE, PA is equal to VA with bits 60~63 cleared, and the cache attribute
+is configured by bits 60~61 in VA: 0 is for strongly-ordered uncached, 1 is
+for coherent cached, and 2 is for weakly-ordered uncached.
+
+Currently we only use XKPRANGE for direct mapping and XSPRANGE is reserved.
+
+To put this in action: the strongly-ordered uncached direct-mapped VA (in
+XKPRANGE) of 0x00000000_00001000 is 0x80000000_00001000, the coherent cached
+direct-mapped VA (in XKPRANGE) of 0x00000000_00001000 is 0x90000000_00001000,
+and the weakly-ordered uncached direct-mapped VA (in XKPRANGE) of 0x00000000
+_00001000 is 0xA0000000_00001000.
+
+Relationship of Loongson and LoongArch
+======================================
+
+LoongArch is a RISC ISA which is different from any other existing ones, while
+Loongson is a family of processors. Loongson includes 3 series: Loongson-1 is
+the 32-bit processor series, Loongson-2 is the low-end 64-bit processor series,
+and Loongson-3 is the high-end 64-bit processor series. Old Loongson is based on
+MIPS, while New Loongson is based on LoongArch. Take Loongson-3 as an example:
+Loongson-3A1000/3B1500/3A2000/3A3000/3A4000 are MIPS-compatible, while Loongson-
+3A5000 (and future revisions) are all based on LoongArch.
+
+.. _loongarch-references:
+
+References
+==========
+
+Official web site of Loongson Technology Corp. Ltd.:
+
+  http://www.loongson.cn/
+
+Developer web site of Loongson and LoongArch (Software and Documentation):
+
+  http://www.loongnix.cn/
+
+  https://github.com/loongson/
+
+  https://loongson.github.io/LoongArch-Documentation/
+
+Documentation of LoongArch ISA:
+
+  https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-Vol1-v1.00-CN.pdf (in Chinese)
+
+  https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-Vol1-v1.00-EN.pdf (in English)
+
+Documentation of LoongArch ELF psABI:
+
+  https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-ELF-ABI-v1.00-CN.pdf (in Chinese)
+
+  https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-ELF-ABI-v1.00-EN.pdf (in English)
+
+Linux kernel repository of Loongson and LoongArch:
+
+  https://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson.git