Subject: Re: Valid use of bus_dma(9)?
To: David Young <email@example.com>
From: Jason Thorpe <firstname.lastname@example.org>
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
>> 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.
-- Jason R. Thorpe <email@example.com>
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