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SubjectRe: New resources - pls, explain :-(


On Fri, 13 Aug 1999, Paul Mackerras wrote:
>
> (I exaggerate. Alan Cox said that not reordering was a safer default
> (which is essentially my position) and Dave Miller said to me in
> private email something along the lines that any sane architecture has
> a side-effect bit in the PTEs to prevent reordering of accesses to
> certain pages.)

Davem looks at it the wrong way. In-order is very much the default for
sane architectures.

A sane architecture has a extra bit that _allows_ re-ordering to certain
areas or pages. It should be off by default, but it might be turned on by
specific drivers. We may add a hint field to "ioremap()" for PCI, for
example, to let PCI devices that know thay can do write combining have a
way to let the CPU know to go wild..

> Richard Henderson mentioned four drivers which he uses which deal with
> this: ncr, aic7xxx, tulip, epic100. How many others would there be in
> this category? It sounds to me like the vast majority of drivers
> which use readl/writel would be assuming that they prevent reordering.

Almost ALL the current drivers depend on not re-ordering. There are
actually very few cases where you can be allowed to re-order widly, and
frame buffers is the only really common case.

Certain other drivers allow a subset of re-ordering which is to allow
collapsing of write events - turning consecutive byte stores into word
stores etc. Not really re-ordering the input or output, but combining
events (works for reads too: a way to tell the hardware that it is ok to
do burst reads over the bus if the pre-fetch logic implies that it makes
sense).

Linus


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