Re: scatter-gather list.

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Mon, 19 Dec 2005, Deven Balani wrote:

> Hi All,
>
> I am trying to port a driver from x86 PCI based platform to ARM platform.
>
> The x86 driver has scatter-gather list as one of the DMA mechanism.
> Can I bypass this, As I believe sg list is for PCI buses and not for
> HSX buses.
>
> Can any one give me the _source_ where I can understand the necessity for
> a PCI bus to use sg list.
>
> Regards,
> deven
> -

The S/G list is used with the INTERFACE chip, i.e., whatever the
device is connected to. If you have a PCI/Bus and the device is
to do DMA across the PCI/Bus with a chip that requires a S/G list,
then you need the S/G list (which is trivial BTW). If your interface
device doesn't require a S/G list then you don't use one.

For instance, a common PCI/Bus interface chip is the PLX PCI 9656BA.
This has a S/G list. In principle, it's possible to bypass the S/G
list, but its downright dumb because you need to get all the information
necessary to build the S/G list anyway, to make one-buffer-at-a-time
DMA (which is what you get without the scatter-list). Typically
scatter lists consists of:

 	uint32_t	padr;	// Bus address to get/put data
 	uint32_t	ladr;	// Local address inside the device
 	uint32_t;	size;	// Transfer size in bytes
 	uint32_t;	dpr; 	// Next descriptor Bus address

That last bus address "dpr" has some low bits that tell the
controller if it is the last in a chain, etc. This stuff needs
to be in physical memory and its bus address is used.

If you don't use a S/G list, then you can't transfer more than
a page of virtual memory because the bus addresses may not
be contiguous. Typically, you build your S/G list using the
appropriate macros to extract the bus address from the virtual
address, so you need this information anyway.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.56 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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