csr_bus: Honour re signal from the upstream bus

Currently CSR bus assumed that ~we means reading, that created
a problem that when for a CSR if reading has side effects and adr
parked unintentionally at that CSR, the reading side effect will be
triggered.

For SoCs, this happened when upstream bus issued a write transaction
with wishbone.sel, then on CSR bus it will be translated
as adr = addr, we = 0, which will be interpreted as a read to such
address, and trigger undesired side effect for such CSR.

Such upstream transaction will be generated by our bus width converter.

Given that we signal already presents in CSR Interface, the easiest way
to handle such situation is to generate re signal at bus bridges and
propagate it all the way down to the Interface.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

litex/soc/interconnect/axi/axi_lite.py

@@ -148,7 +148,7 @@ class AXILiteRemapper(LiteXModule):
 
 # AXI-Lite to Simple Bus ---------------------------------------------------------------------------
 
-def axi_lite_to_simple(axi_lite, port_adr, port_dat_r, port_dat_w=None, port_we=None):
+def axi_lite_to_simple(axi_lite, port_adr, port_dat_r, port_dat_w=None, port_re=None, port_we=None):
     """Connection of AXILite to simple bus with 1-cycle latency, such as CSR bus or Memory port"""
     bus_data_width = axi_lite.data_width
     adr_shift      = log2_int(bus_data_width//8)
@@ -168,6 +168,9 @@ def axi_lite_to_simple(axi_lite, port_adr, port_dat_r, port_dat_w=None, port_we=
         else:
             comb.append(port_we.eq(axi_lite.w.valid & axi_lite.w.ready & (axi_lite.w.strb != 0)))
 
+    if port_re is not None:
+        comb.append(port_re.eq(axi_lite.ar.valid & axi_lite.ar.ready))
+
     port_adr_reg = Signal(len(port_adr))
 
     fsm = FSM()

litex/soc/interconnect/axi/axi_lite_to_csr.py

@@ -32,6 +32,7 @@ class AXILite2CSR(LiteXModule):
         fsm, comb = axi_lite_to_simple(
             axi_lite   = self.axi_lite,
             port_adr   = self.csr.adr,
+            port_re    = self.csr.re,
             port_dat_r = self.csr.dat_r,
             port_dat_w = self.csr.dat_w,
             port_we    = self.csr.we)

litex/soc/interconnect/csr_bus.py

@@ -18,9 +18,11 @@ sys_clk domain of the SoC, completing writes in a single cycle and reads in two
                          ┌───────────┐                  Write in 1 cycle:
                          │           │                  - adr/we/dat_w set by bridge.
                          │           ├───► adr
-                         │           │                  Read in 2 cycles:
+                         |           |                  Read in 2 cycles:
-       Main SoC Bus ◄────►    CSR    ├───► we           - adr set by bridge
+                         │           ├───► re           - adr and re set by bridge.
-                         │   Bridge  │                  - dat_r set returned by user logic.
+       Main SoC Bus ◄────►           │                  - User logic can ignore re if there is no reading side effect.
+                         |    CSR    ├───► we           - dat_r set returned by user logic.
+                         │   Bridge  │
                          │           ├───► dat_w
                          │           │
                          │           ◄──── dat_r
@@ -46,6 +48,7 @@ from litex.soc.interconnect.csr import CSRStorage
 
 _layout = [
     ("adr",  "address_width", DIR_M_TO_S),
+    ("re",                 1, DIR_M_TO_S),
     ("we",                 1, DIR_M_TO_S),
     ("dat_w",   "data_width", DIR_M_TO_S),
     ("dat_r",   "data_width", DIR_S_TO_M)
@@ -81,9 +84,11 @@ class Interface(Record):
 
     def read(self, adr):
         yield self.adr.eq(adr)
+        yield self.re.eq(1)
         yield
-        yield
+        value = (yield self.dat_r)
-        return (yield self.dat_r)
+        yield self.re.eq(0)
+        return value
 
 # CSR Interconnect ---------------------------------------------------------------------------------
 
@@ -97,6 +102,7 @@ class InterconnectShared(Module):
         intermediate = Interface.like(masters[0])
         self.comb += [
             intermediate.adr.eq(  Reduce("OR", [masters[i].adr   for i in range(len(masters))])),
+            intermediate.re.eq(   Reduce("OR", [masters[i].re    for i in range(len(masters))])),
             intermediate.we.eq(   Reduce("OR", [masters[i].we    for i in range(len(masters))])),
             intermediate.dat_w.eq(Reduce("OR", [masters[i].dat_w for i in range(len(masters))]))
         ]
@@ -207,8 +213,8 @@ class CSRBank(csr.GenericBank):
             self.comb += [
                 c.r.eq(self.bus.dat_w[:c.size]),
                 If(sel & (self.bus.adr[:log2_int(aligned_paging)] == i),
-                    c.re.eq( self.bus.we),
+                    c.re.eq(self.bus.we),
-                    c.we.eq(~self.bus.we)
+                    c.we.eq(self.bus.re)
                 )
             ]
 

litex/soc/interconnect/wishbone.py

@@ -602,12 +602,14 @@ class Wishbone2CSR(LiteXModule):
                 NextValue(self.csr.dat_w, self.wishbone.dat_w),
                 If(self.wishbone.cyc & self.wishbone.stb,
                     NextValue(self.csr.adr, self.wishbone.adr[wishbone_adr_shift:]),
+                    NextValue(self.csr.re, ~self.wishbone.we & (self.wishbone.sel != 0)),
                     NextValue(self.csr.we,  self.wishbone.we & (self.wishbone.sel != 0)),
                     NextState("WRITE-READ")
                 )
             )
             fsm.act("WRITE-READ",
                 NextValue(self.csr.adr, 0),
+                NextValue(self.csr.re, 0),
                 NextValue(self.csr.we, 0),
                 NextState("ACK")
             )
@@ -623,7 +625,8 @@ class Wishbone2CSR(LiteXModule):
                 self.csr.dat_w.eq(self.wishbone.dat_w),
                 If(self.wishbone.cyc & self.wishbone.stb,
                     self.csr.adr.eq(self.wishbone.adr[wishbone_adr_shift:]),
-                    self.csr.we.eq(self.wishbone.we & (self.wishbone.sel != 0)),
+                    self.csr.re.eq(~self.wishbone.we & (self.wishbone.sel != 0)),
+                    self.csr.we.eq( self.wishbone.we & (self.wishbone.sel != 0)),
                     NextState("ACK")
                 )
             )

test/test_csr.py

@@ -19,9 +19,9 @@ def csr32_write(dut, adr, dat):
 
 def csr32_read(dut, adr):
     dat = 0
-    for i in range(4):
+    for i in range(5):
         dat |= ((yield from dut.csr.read(adr + 3 - i)) << 8*i)
-    return dat
+    return dat >> 8
 
 
 class CSRModule(Module, csr.AutoCSR):