Scatter Gather List Questions

Scatter Gather List Questions

[Craig Rich]

Jeff:

	Hello. Saw this post on the Internet, and was hoping you are still open 
to answering some questions about scatter/gather lists in Linux.

	I'm tyring to update the Linux driver for our triple speed Ethernet MAC 
(we haven't formally given the driver to you yet; I guess we should soon.)

	Anyhow, I'm trying to find documentation, or examples of how to use the 
scatter/gather capabilities of Linux, and not having much luck. So 
basically, I'm not sure where to start. I tried grepping the source as 
you say below, its not helping much.

	One question I can start with is, how do you use pci_dma_sg?

pci_map_sg(dev, sglist, nents, direction);

	I'm assuming I supply the dev and direction fields. How about the nents 
field? Is that supposed to be the largest number of fragments I can 
handle (that's what I assumed.) Finally, the sglist argument has me 
really confused. Do I have to create this structure in advance (and if 
so how) or is pci_map_sg supposed to simply give me a pointer back via 
the sglist argument (that's what I assumed, but that doesn't seem to be 
the case unless I'm doing something else wrong.)

	Also, were in the source code is pci_map_sg located? I'll admit I'm not 
an expert at looking through the source code of an OS like Linux, but 
I'm frustrated by the fact that a simple grep of /usr/src/linux-2.4.2 
does not show where this function is coded.

	Any help would be greatly appreciated.

> Jeff Garzik  linux-kernel@vger.kernel.org
> Wed, 14 Nov 2001 20:41:34 -0500
> 
>     * Previous message: [linux-kernel] Need Info on Checksum Offloading
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>     * Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
> 
> harish.vasudeva@amd.com wrote:
>> > Hi All,
>> > Could any1 pls direct me wherein i could find some documentation about implementing checksum offloading for my ethernet LAN driver?
> 
> 
> There is not good documentation.  Feel free to ask me questions, I am
> the Linux network driver maintainer.
> 
> First, search the source code in linux/drivers/net/* for
> NETIF_F_IP_CSUM, NETIF_F_HW_CSUM, skb->csum, and nr_frags.
> 
> Can I expect an ethernet driver submitted to me, soon?
> 
> 	Jeff
> 
> -- 
> Jeff Garzik      | Only so many songs can be sung
> Building 1024    | with two lips, two lungs, and one tongue.
> MandrakeSoft     |         - nomeansno
> 
> 


-- 
Craig Rich
Senior Systems Engineer         mailto:craig_rich@sundanceti.com
Sundance Technology Inc.        http://www.sundanceti.com
1485 Saratoga Avenue            tel: 408 873 4117 x107
Suite 200                       fax: 408 996 7064
San Jose, CA, USA  95129

Re: Scatter Gather List Questions

[Jens Axboe]

On Thu, Feb 07 2002, Craig Rich wrote:
> 	One question I can start with is, how do you use pci_dma_sg?
> 
> pci_map_sg(dev, sglist, nents, direction);
> 
> 	I'm assuming I supply the dev and direction fields. How about the nents 

Of course, dev being your pci device and direction the data direction --
PCI_DMA_TODEVICE etc, see linux/pci.h.

> field? Is that supposed to be the largest number of fragments I can 
> handle (that's what I assumed.) Finally, the sglist argument has me 

nents is the number if segments in the sglist you are supplying as well.
not the maximum number of entries the driver can handle.

> really confused. Do I have to create this structure in advance (and if 
> so how) or is pci_map_sg supposed to simply give me a pointer back via 
> the sglist argument (that's what I assumed, but that doesn't seem to be 
> the case unless I'm doing something else wrong.)

No you have to allocate this structure yourself.

> 	Also, were in the source code is pci_map_sg located? I'll admit I'm not 
> an expert at looking through the source code of an OS like Linux, but 
> I'm frustrated by the fact that a simple grep of /usr/src/linux-2.4.2 
> does not show where this function is coded.

You are not terribly good a grepping, it seems :-). The function is in
asm/pci.h, depending on the architecture you may have to look inside
arch/ for helpers too.

-- 
Jens Axboe