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Re: Linux mips and DMA

To: Ralf Baechle <>
Subject: Re: Linux mips and DMA
From: Jon Dufresne <>
Date: Fri, 11 Jan 2008 11:08:00 -0500
In-reply-to: <>
Original-recipient: rfc822;
References: <> <> <> <> <> <>
> > > Hardware coherency for DMA is the exception for low-end embedded MIPS
> > > systems andgiven the CPU address your's is no exception from that.
> > > 
> > > If your system was supporting hardware coherency for DMA I/O you would
> > > have obtained a cachable CPU address like:
> > > 
> > >   dma_handle=0x026f0000 size=0x00010000 cpu_addr=0x826f0000
> > >                                                   ^^^
> > > 
> > > A 0x8??????? would be in KSEG0 so cachable.
> > 
> > I do have a an embedded system. Are you saying that, in all likelyhood,
> > I do not have coherency? If I understand you correctly, this is a bad
> > thing right? Will I need to take extra care to work around this issue.
> > 
> > So are you saying I would prefer a cpu_addr in the 0x8******* range?
> No. because you don't seem to have hw coherency.

So, the address I received is in uncachable memory, and therefore,
should look the same to all devices on the bus without hw cache

> > If it is true that I don't have hardware coherency should I still be
> > using the pci_*_consistent api? Or should I switch to the
> > dma_*_noncoherent api? Also what extra steps do I need to take to get
> > this to work with a non-coherent system?
> What you were doing seemed to be the right thing.  The API is supposed
> to do the necessary address conversion and cache flushes for the driver.
> That is the unchanged driver should work on any architecture.

Should I need to do any explicit cache flushes in my code for hardware
without cache coherency? If I wanted to do this what function should I

> According to the current kernel code the PNX8550 non-coheren (aka software
> coherency).

Will this be a problem for what I'm trying to accomplish? That is dma on
a pci device.


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