Subject: bus_dmamap_sync not enough ?
To: None <firstname.lastname@example.org>
From: Stephan Uphoff <email@example.com>
Date: 02/11/2003 00:54:03
bus_dmamap_sync not enough for some devices ?
Some PCI devices use DMA to send command completion and status information to
(Example Intel Pro 100 - sys/dev/ic/i82257.c )
Communication between driver and device borrows tactics from SMP environments.
However, bus_dmamap_sync is not suited for this kind of communication, since
both device and
CPU(s) need to have access to the buffer at the same time.
What I think is missing to implement this correctly are machine-independent
functions to access
main memory in a strongly ordered way.
(Just declaring a structure member as volatile is not enough - in SMP the
provide mutual exclusion AND strong memory ordering through memory barriers.)
I can attest to the need for ordered memory access.
A few years ago, I personally tracked down out of order CPU reads of ordered
82257 DMA writes
on a proprietary 82257 Ethernet driver, running on a Pentium III machine.
(Just a few cases per day on stress tests - CPU reads just a few opcodes
Is there a clean machine-independent way to handle these devices on netbsd
that I missed ?
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