Martin (1992: 117-8):
The final comparison system to be considered relates participants to relevant proposals (see Halliday 1985: 235-47) and is realised by too and enough. These items assess quantity or quality with respect to the amount or degree necessary to enable a presumed proposal; this function is perhaps clearest when the proposal is embedded in the same nominal group as in [3:42] and [3:43]:
[3:42] The boy was too upset [[to eat his breakfast]].
[3:43] The boy had enough time [[to finish it]].
Too realises [difference]; [3:42] means that the boy was upset to a degree different from that required to eat his breakfast. Enough on the other hand realises [semblance]; [3:43] means that the boy had the amount of time identical to that he needed to finish.
 To be clear, contrary to the implication, Halliday (1985: 235-47) says nothing about comparison, and does not endorse the theoretical misunderstandings presented here by Martin.
 To be clear, in modifying the Head of a nominal group in terms of modality — possibility, necessity — these grammatical items function interpersonally.
 To be clear, neither of these embedded non-finite clauses realises a proposal (command or offer); both would realise propositions (statements) as ranking clauses: the boy ate his breakfast, the boy finished it.
 To be clear, this is 'difference' in the most general sense of one semiotic feature being different from another.
 To be clear, too in [3:42] means 'to a higher degree than is possible' (for eating breakfast). It does not function as a comparative reference item because it makes no comparison of upset with to eat his breakfast.
 To be clear, this misunderstands the meaning of 'semblance'. In English, 'semblance' means 'the outward appearance or apparent form of something, especially when the reality is different'.
 To be clear, enough in [3:43] serves the same function as sufficient: a post-Deictic of modulation, as shown by the agnates (the) required and (the) necessary. It does not function as a comparative reference item because it makes no comparison of time with to finish it.