newbus IO ordering semantics - moving forward

[Adrian Chadd]

On 28 October 2011 15:37, John-Mark Gurney <jmg at funkthat.com> wrote:
>> I'd appreciate some feedback/comments before I go off and code all of this up.
>
> I think we should complain about the drivers that are NOT using the
> lazy/loose semantics as those are the drivers that are slower than
> they should be, and/or not written properly.  Complaining about properly
> written drivers that use the lazy/loose semantics when they get updated
> to be correct is wrong...

Right. My point though is that I'm not sure of how many drivers have
been tested outside of i386/amd64. Marcus's response is helpful,
indicating that the sparc64 guys have tested a lot of this. Yes, the
barrier calls are expensive and yes, drivers on i386/amd64 still need
to do bus_dmamap_sync() calls.

I can only speak from my limited experience here after tracking down
that ath/ath_hal bug. My experience is that ath(4) triggered on PPC
because of a loop which read the same register a few times. I recall
seeing an ethernet driver recently have a commit which also did this.
I'm happy to do the reverse. Ie, on platforms where it matters, add a
warning printf (in verbose boot) when drivers aren't indicating
they've been fully tested. Or, I'm happy to completely ignore the
situation. :)



Adrian