Subject: Re: Valid use of bus_dma(9)?
To: None <>
From: Matt Thomas <>
List: tech-kern
Date: 07/23/2004 16:04:54
At 03:34 PM 7/23/2004, you wrote:

>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 
>>>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.

which fpa does.  It allocates a 8KB chunk of which only a small portion
should be coherent.  it try to map/load this small chunk as coherent which
is what that code is trying to do.

Matt Thomas                     email:
3am Software Foundry              www:
Cupertino, CA              disclaimer: I avow all knowledge of this message.