Fragmenter: add a third case for earlyAck (PutFulls only)
It seems quite common to have a device that is backed by ECC. When
performing a multibeat PutPartial, these devices can exhibit their
first error on the last beat (if it had an incomplete write mask
for that beat, which required read-write-modifying corrupted data).
Generally, these devices have ECC granularity <= the bus width. In
those cases, if you send a PutFull, the first beat carries the
error value for the whole burst. Consider:
If the PutFull was below the granularity, it was a single beat.
If the PutFull was multi-beat, it exceeds the granularity.
Therefore, an important variation on the earlyAck optimization is
the case where only PutFulls receive an earlyAck.
When we first implemented TL, we thought this was helpful, because
it made WidthWidgets stateless in all cases. However, it put too
much burden on all other masters and slaves, none of which benefitted
from this signal. Furthermore, even with addr_lo, WidthWidgets were
information lossy because when they widen, they have no information
about what to fill in the new high bits of addr_lo.
Consider the following waveform for two 4-beat bursts:
---A----A------------
-------D-----DDD-DDDD
Under TL rules, the second A can use the same source as the first A,
because the source is released for reuse on the first response beat.
However, if we fragment the requests, it looks like this:
---3210-3210---------
-------3-----210-3210
... now we've broken the rules because 210 are twice inflight.
To solve this, we alternate an a.source bit every time D completes a txn.
Wesley W. Terpstra
Henry Cook
Megan Wachs