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Fix AXI4 FIFO ordering for masters with early source reuse (#1108)

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* TLToAXI4: fix WaR for single-source FIFO masters
* TLToAXI4: fix potential counter overflow => WaR hazard

If you have a FIFO master with 2^n-1 sources that performs early
source re-use, the old code could potentially break FIFO order.
This commit is contained in:
Wesley W. Terpstra 2017-11-13 20:32:09 -08:00 committed by GitHub
parent 7098ebf439
commit 147fad6387
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1 changed files with 11 additions and 4 deletions
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15
src/main/scala/tilelink/ToAXI4.scala
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@ -191,8 +191,15 @@ class TLToAXI4(val combinational: Boolean = true, val adapterName: Option[String
val d_sel = UIntToOH(Mux(r_wins, out.r.bits.id, out.b.bits.id), edgeOut.master.endId).toBools
val d_last = Mux(r_wins, out.r.bits.last, Bool(true))
// If FIFO was requested, ensure that R+W ordering is preserved
(a_sel zip d_sel zip idStall zip idCount) filter { case (_, n) => n.map(_ > 1).getOrElse(false) } foreach { case (((as, ds), s), n) =>
val count = RegInit(UInt(0, width = log2Ceil(n.get + 1)))
(a_sel zip d_sel zip idStall zip idCount) filter { case (_, n) => n.isDefined } foreach { case (((as, ds), s), n) =>
// AXI does not guarantee read vs. write ordering. In particular, if we
// are in the middle of receiving a read burst and then issue a write,
// the write might affect the read burst. This violates FIFO behaviour.
// To solve this, we must wait until the last beat of a burst, but this
// means that there can be idCount+1 operations counted due to a TileLink
// master which performs early source reuse.
val maxCount = n.get + 1
val count = RegInit(UInt(0, width = log2Ceil(maxCount + 1)))
val write = Reg(Bool())
val idle = count === UInt(0)
@ -200,8 +207,8 @@ class TLToAXI4(val combinational: Boolean = true, val adapterName: Option[String
val dec = ds && d_last && in.d.fire()
count := count + inc.asUInt - dec.asUInt
assert (!dec || count =/= UInt(0)) // underflow
assert (!inc || count =/= UInt(n.get)) // overflow
assert (!dec || count =/= UInt(0)) // underflow
assert (!inc || count =/= UInt(maxCount)) // overflow
when (inc) { write := arw.wen }
s := !idle && write =/= arw.wen