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RE: BCM91x80A/B PCI DMA problems

To: "Martin Michlmayr" <tbm@cyrius.com>
Subject: RE: BCM91x80A/B PCI DMA problems
From: "Mark E Mason" <mark.e.mason@broadcom.com>
Date: Tue, 14 Mar 2006 08:46:05 -0800
Cc: linux-mips@linux-mips.org
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
Thread-index: AcZHhcQE4UFfAMQeRH2SJj413qubvgAABssQ
Thread-topic: BCM91x80A/B PCI DMA problems
Hello, 

> * Mark E Mason <mark.e.mason@broadcom.com> [2006-03-14 08:21]:
> > There's a lot of PCI HW out there that's 32-bit only, but 
> there's also 
> > a fair bit that isn't.  A 32-bit device in a 64-bit system 
> is going to 
> > require bounce buffering by the driver/OS.
> 
> If I understand you correctly, you're saying that you 
> currently don't supporting 32 bit PCI cards on the SB1 
> platform.

Um - not exactly.  A 32-bit device in a 64-bit system will require some
form of bounce buffering.   This is true regardless of what the platform
is, or what the OS/RTOS is.

Sun did some interesting stuff with this in the early UltraSPARC systems
with a 'reverse I/O MMU' which bypassed the need for bounce buffering
but imposed other requirements on the drivers and OS.  Of course, that
required specialized HW which doesn't seem to be in vogue.

>  Is this something you care about, or should I get 
> a 64-bit PCI card instead?  How hard would it be to implement 
> bounce buffering anyway?

This is something I care about.

How difficult it is under Linux - I (personally) don't know.  Based on
other posts, it sounds like Linux has a system for it... But like I said
earlier, I'm far more familiar with certain other RTOS/firmware
environments than I am Linux.

If someone more familiar with this area could point me in the right
direction though, I'll get someone to look at it (if I don't myself).

Thanks,
Mark

> --
> Martin Michlmayr
> http://www.cyrius.com/
> 
> 


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