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From: Kunihiko IMAI <kimai@iodata.jp>
To: linux-mips@linux-mips.org
Subject: Re: USB on MIPS
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Hi,

At Thu, 18 Dec 2003 09:43:21 -0800,
Jun Sun wrote:
> 
> On Thu, Dec 18, 2003 at 07:58:36PM +0530, samavarthy c wrote:
> > Hi,
(snip)
> 
> Another possiblity (which is probably more likely) is the IRQ
> number is not seupt correctly.  

And another possibilities are:

	DMA cache coherency
		NEC VR4xxxs have no bus-snoop function. So
		cache-coherency must be maintaind by software.
		(or use KSEG1 access)

		But I remember linux-2.4.18 of usb-ohci.c doesn't have
		this problem.

	Read/write cycle order
		Read/write cycle order of CPU instruction level is
		guaranteed at bus level?
		( Or at assembler level, the sequent instruction is
		re-ordered ? )

		Newer version of usb-ohci.c has some wmb() funtions.
		It synchronizes read/write order.

I got similar errors on VR4181A with 2.4.18 kernel.  VR4181A has
built-in USB OHCI 1.1 host connected internal PCI bus.

To fix this problem, I compared newer usb-ohci.c and put wmb()
corresponded place. It is expaneded as subsequent NOPs. (VR4xxx has
real SYNC instruction.)

Thanks.
_._. __._  _ . ... _  .___ ._. _____ _... ._ _._ _.._. .____  _ . ... _

                                                          Kunihiko IMAI