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From: Warner Losh <imp@bsdimp.com>
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To: Hans Petter Selasky <hselasky@c2i.net>
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Cc: Adrian Chadd <adrian@freebsd.org>, freebsd-arch@freebsd.org
Subject: Re: newbus IO ordering semantics - moving forward
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On Oct 28, 2011, at 1:15 AM, Hans Petter Selasky wrote:

> On Thursday 27 October 2011 22:28:13 Adrian Chadd wrote:
>> * Make the bus default to use ordered semantics, much like what Linux
>> does - ie, all IO read/writes (io or memory) are in-order and flushed
>> with a barrier;
>=20
> I think this is the assumption of many USB controller drivers =
currently in the=20
> tree.

The ordering guarantees aren't as random as Adrian suggests.  All writes =
are posted before any reads is about all you can count on.  Most drivers =
naturally cope with this well.  All reads have to be synchronous by =
their very nature and in-order.  There's more liberty taken with writes =
wrt ordering.

Where it falls down is DMA or MSI.  Both of those have writes from the =
device to the system memory, and ordering of the completion of those =
relative to other things isn't completely guaranteed.

Warner