“all that” vs. “all what”

I’ve heard somebody say:

All what is needed is …

I thought the correct way to phrase it was:

All that is needed is …

However, thinking about it more, the former doesn’t sound too incorrect, albeit a bit odd. Is the former grammatically correct? Am I alone in thinking the latter sounds more natural?

I'd say all what is needed is at best semi-literate. True, Google Books give over 50,000 written instances, but probably most of them are "accidental collocations" (e.g. - a sentence ending with the word "all", followed by one starting with "What is needed"). Also note that this contrasts with well over 3 million instances of all that is needed.

In some dialects of English, 'what' is used instead of 'that' as a generic relative clause.

The car what I saw this morning...

is non-standard but works in (I can't find a reference but Estuary or Cockney sounds right to me).

All what is needed...

may be a similar usage of 'what' instead of 'that' for that person's dialect.

If written as All wot is needed, I could pass it off as dialectic, but in modern English all that is far more common.

All which seems to be right out.

'All what is needed' = 'All that which is needed', substituting [ what with that which ].

Not necessarily ungrammatical nor odd usage. Could be uncommon, though.

It appears, on the whole, to be an archaic usage, but here’s an OED citation from 2007:

The Riemann tensor tells us all what we need to know about the intrinsic curvature and thus gravity.

Practical advice might well be to use all that, but if you want to use all what, then there are many precedents.

EDIT:

The OED’s definition 7 of what is ‘As simple relative (sing. or pl.): Which (or who); that.’ Within that, the first sub-category reads:

a. referring to a pronoun (demonstrative or indefinite), occasionally to a noun; originally introducing an indirect question in apposition to it especially, in later use only, in all what (now dialect or vulgar).

There are 14 supporting citations under this entry. There are others elsewhere in the OED illustrating all what, including one from ‘Timon of Athens’.