dcache: Add more commentary, no code change

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Paul Mackerras 3 years ago
parent 55f7d99376
commit 784d409999

@ -1,12 +1,6 @@
-- Set associative dcache write-through
-- TODO (in no specific order):
-- * See list in icache.vhdl
-- * Complete load misses on the cycle when WB data comes instead of
-- at the end of line (this requires dealing with requests coming in
-- while not idle...)
library ieee;
use ieee.std_logic_1164.all;
@ -57,7 +51,7 @@ end entity dcache;
architecture rtl of dcache is
-- BRAM organisation: We never access more than wishbone_data_bits at
-- a time so to save resources we make the array only that wide, and
-- use consecutive indices for to make a cache "line"
-- use consecutive indices to make a cache "line"
-- ROW_SIZE is the width in bytes of the BRAM (based on WB, so 64-bits)
constant ROW_SIZE : natural := wishbone_data_bits / 8;
@ -206,20 +200,80 @@ architecture rtl of dcache is
-- which means that the BRAM output is delayed by an extra cycle.
-- Thus, the dcache has a 2-stage internal pipeline for cache hits
-- with no stalls.
-- with no stalls. Stores also complete in 2 cycles in most
-- circumstances.
-- A request proceeds through the pipeline as follows.
-- Cycle 0: Request is received from loadstore or mmu if either
-- d_in.valid or m_in.valid is 1 (not both). In this cycle portions
-- of the address are presented to the TLB tag RAM and data RAM
-- and the cache tag RAM and data RAM.
-- Clock edge between cycle 0 and cycle 1:
-- Request is stored in r0 (assuming r0_full was 0). TLB tag and
-- data RAMs are read, and the cache tag RAM is read. (Cache data
-- comes out a cycle later due to its output register, giving the
-- whole of cycle 1 to read the cache data RAM.)
-- Cycle 1: TLB and cache tag matching is done, the real address
-- (RA) for the access is calculated, and the type of operation is
-- determined (the OP_* values above). This gives the TLB way for
-- a TLB hit, and the cache way for a hit or the way to replace
-- for a load miss.
-- Clock edge between cycle 1 and cycle 2:
-- Request is stored in r1 (assuming r1.full was 0)
-- The state machine transitions out of IDLE state for a load miss,
-- a store, a dcbz, or a non-cacheable load. r1.full is set to 1
-- for a load miss, dcbz or non-cacheable load but not a store.
-- Cycle 2: Completion signals are asserted for a load hit,
-- a store (excluding dcbz), a TLB operation, a conditional
-- store which failed due to no matching reservation, or an error
-- (cache hit on non-cacheable operation, TLB miss, or protection
-- fault).
-- For a load miss, store, or dcbz, the state machine initiates
-- a wishbone cycle, which takes at least 2 cycles. For a store,
-- if another store comes in with the same cache tag (therefore
-- in the same 4k page), it can be added on to the existing cycle,
-- subject to some constraints.
-- While r1.full = 1, no new requests can go from r0 to r1, but
-- requests can come in to r0 and be satisfied if they are
-- cacheable load hits or stores with the same cache tag.
-- All other operations are handled via stalling in the first stage.
-- Writing to the cache data RAM is done at the clock edge
-- at the end of cycle 2 for a store hit (excluding dcbz).
-- Stores that miss are not written to the cache data RAM
-- but just stored through to memory.
-- Dcbz is done like a cache miss, but the wishbone cycle
-- is a write rather than a read, and zeroes are written to
-- the cache data RAM. Thus dcbz will allocate the line in
-- the cache as well as zeroing memory.
-- The second stage can thus complete a hit at the same time as the
-- first stage emits a stall for a complex op.
-- Since stores are written to the cache data RAM at the end of
-- cycle 2, and loads can come in and hit on the data just stored,
-- there is a two-stage bypass from store data to load data to
-- make sure that loads always see previously-stored data even
-- if it has not yet made it to the cache data RAM.
-- Load misses read the requested dword of the cache line first in
-- the memory read request and then cycle around through the other
-- dwords. The load is completed on the cycle after the requested
-- dword comes back from memory (using a forwarding path, rather
-- than going via the cache data RAM). We maintain an array of
-- valid bits per dword for the line being refilled so that
-- subsequent load requests to the same line can be completed as
-- soon as the necessary data comes in from memory, without
-- waiting for the whole line to be read.

-- Stage 0 register, basically contains just the latched request
type reg_stage_0_t is record
req : Loadstore1ToDcacheType;
tlbie : std_ulogic;
doall : std_ulogic;
tlbld : std_ulogic;
tlbie : std_ulogic; -- indicates a tlbie request (from MMU)
doall : std_ulogic; -- with tlbie, indicates flush whole TLB
tlbld : std_ulogic; -- indicates a TLB load request (from MMU)
mmu_req : std_ulogic; -- indicates source of request
end record;