Re: _clear_page semantics

On Tue, Jan 09, 2001 at 05:48:11PM -0800, Justin Carlson wrote:

Looking at the existing clear_page implementations for r4xx0, rm7k, and mips32
in the mips/ tree, I see everyone issuing cache op 0xd for the address range of
the page being cleared.

I'm wondering what the purpose is of these cache flushes...given a physically
tagged dcache, my understanding of the semantics of clear_page are that it just
zeros the page, in which case the cache ops are pointless overhead.

Especially in the mips32 case, which uses cache op 0xd, which is undefined
implementation dependent according to my mips32 spec.

The idea is to avoid unnecessary memory reads - all the read data would
be overwritten anyway.  The last time I benchmarked this routine on some
machine it made a difference of about 4000 vs. 2500 c0_count cycles.  I
think that was on a R4600 RM200.

  Ralf