Messages in this thread | | | Subject | Re: scatter-gather list. | Date | Tue, 20 Dec 2005 09:06:49 -0500 | From | "linux-os \(Dick Johnson\)" <> |
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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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