CPU caching of flash regions.

[David Woodhouse <dwmw2@infradead.org>]

I've just seen profiling of a system mounting JFFS2 filesystem which shows 
that the majority of the time is spend in the map driver's copy_from 
function.

The copy_from() functions are currently using a completely uncached mapping 
of the flash chip, but in fact for reading the chip that's not strictly 
necessary. This is especially true during the initial scan. 

I think we ought to allow map drivers to do intelligent caching of bus 
accesses. Suggested semantics:

 1. Only the copy_from() and copy_to() functions can use a cacheable mapping.

 2. Any access to the chip through one of the other ({read,write}{8,16,32}) 
	functions causes the cache to be flushed for the entire mapping.

If a cache flush is expensive, a mapping driver may optimise the flushes and
perform a cache flush only if the cache is expected to be non-empty.

This approach is fairly simple, and allows mapping drivers to do something 
closely approximating the "right thing" without adding complexity to the 
chip driver code. An alternative, which I'm dubious about, is to add 
explicit cache management functionality to the methods exported by the 
mapping drivers, and to have the chip driver explicitly turn the cache 
on/off and flush parts of it when writing/erasing.

Comments?

--
dwmw2