Subject: Re: Valid use of bus_dma(9)?
To: David Young <>
From: Jason Thorpe <>
List: tech-kern
Date: 07/23/2004 15:34:13
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On Jul 23, 2004, at 1:38 PM, David Young wrote:

> On Fri, Jul 23, 2004 at 05:19:23PM +0200, Manuel Bouyer wrote:
>> bus_dmamem_alloc(9) says:
>>             The mapping of this memory is machine-dependent (or 
>> "opaque");
>> 	    machine-independent code is not to assume that the addresses
>> 	    returned are valid in kernel virtual address space, or that the
>> 	    addresses returned are system physical addresses.  The address
>> To me, this means that we can't make any assumptions about the values 
>> returned
>> by bus_dmamem_alloc(), and especially it's wrong to assume we can do
>> memory-related operations on ds_addr/ds_len.
> Network drivers don't copy ds_addr into transmit/receive descriptors?
> How does this work if they are not "system physical addresses" ?

System physical addresses != DMA addresses.  At least, on many 
platforms this is the case.

The whole idea is that bus_dmamem_alloc() returns addresses that are 
opaque to the driver.  The driver MUST use bus_dmamap_load*() in order 
to translate an address to one valid for use in DMA.

         -- Jason R. Thorpe <>

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