DMA Failure: 2.6.10-gentoo-r5 on PIIX Chipset [SOLVED!]

Bob P · Advocate Joined: 20 Oct 2004 Posts: 3374 Location: USA

The Gentoo Development Sources 2.6.10-r5 kernel is kicking my ass. I've got a P133 that I'd like to turn into a router. Unfortunately, with the current stable branch kernel, I can't get DMA to work on the machine.

lspci:

gsurbey · Posted: Fri Jan 21, 2005 3:10 am Post subject:

Your DMA not working almost always has to do with the specific IDE controller driver not being enabled in the kernel. Just having "generic/default IDE chipset support" does not do it for DMA. Make sure to have "Use PCI DMA by default when available"

Run "lspci | grep IDE" (part of the sys-apps/pciutils package)
The output should look similar to mine except have your controller name:

dsd · Posted: Fri Jan 21, 2005 3:33 am Post subject:

output of "lspci -n" would also be useful
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Bob P · Advocate Joined: 20 Oct 2004 Posts: 3374 Location: USA

yeah, i already checked the obvious things. :wink:

as you have recommended, I have set the following settings to true:

Bob P · Advocate Joined: 20 Oct 2004 Posts: 3374 Location: USA

dsd, thanks for the tip. i'll have to write "lspci-n" down on my list of things that i can't seem to remember. :wink:

what specifically does the output from lspci -n tell you?

lspci

gsurbey · Posted: Fri Jan 21, 2005 4:30 am Post subject:

lol, it happens but I'm glad you solved it. I'm currious how lspci -n helps too. Do you grep -Hr "some number" /usr/src/linux/ to find the driver or something?
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dsd · Posted: Fri Jan 21, 2005 12:37 pm Post subject:

if you know the kernel source you can look up which devices are supported. yours is supported by the piix driver but does not support piix DMA, so it is falling back to generic IDE DMA for you (i guess this is why it does not come on automatically)
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Bob P · Advocate Joined: 20 Oct 2004 Posts: 3374 Location: USA

this is one one of the great mysteries that i've never understood in Gentoo -- How is it that a Genuine Intell PIIX chipset is not supported by the linux PIIX DMA?

I mean, what good is the linux PIIX driver if it doesn't support the genuine Intel PIIX chipset tht is the model upon which all similar chipsets are modeled? This makes absolutely no sense. :?:

dsd · Posted: Sat Jan 22, 2005 1:26 am Post subject:

the piix driver does support your hardware. it supports it just fine. your disk is working in linux, what more do you want? infact, if you didnt have the PIIX driver compiled in, you'd probably notice some performance loss or lack of features when querying for smart logs and similar.

oh right, you want UDMA. well, your hardware(ide controller) apparently doesnt support UDMA, so you cant have it!

your ide controller does have *some* hardware capabilities though. this allows linux to enable "MWDMA", which (i think) is MiddleWare DMA, i.e. partially implemented in hardware, partially implemented in software. (udma is implemented almost fully in hardware)

the piix driver knows that your device doesnt do UDMA, so it tells the kernel to fall back to MWDMA. mwdma is provided by the generic ide driver. and it lets you have mwdma, if you really want it

[ i'm not 100% sure that the above is fully correct, i'm not an expert here myself. but i think this is fairly accurate as to what is going on. ]

edit : a clarification of what DMA is might help you understand the scenario here. before DMA existed, the CPU would ask the disk to read a few sectors. the disk would read a byte, send a in interrupt to the CPU, the CPU would then write the byte into memory. the disk would then get another byte, generate another interrupt, etc.
you can imagine, as disks got faster, this resulted in more and more CPU time being spent upon writing small amounts of data into memory. CPU speed started becoming the bottleneck for disk transfer bandwidth, not to mention that the CPU was too busy to serve your other applications when disk activity was taking place, so you got considerably system "lag".
so DMA came along.. this is where the CPU asks the disk to read some data, and to write it into a specific memory location. the hard disk then reads the data and writes it directly into memory, without involving the CPU. once the data is written (usually a few megabytes or so, i guess), it sends an interrupt to the CPU to inform it that the transfer is complete.

as a downside, implementing DMA basically means you have to implement a memory manager in your IDE device. i guess this adds a fair amount of hardware to the controller, perhaps why not all of them support dma, or why they dont support it in full without a lot of help from software.

does this make it any clearer?
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Bob P · Advocate Joined: 20 Oct 2004 Posts: 3374 Location: USA

dsd · Posted: Sat Jan 22, 2005 10:12 pm Post subject:

the point i was trying to make is that DMA is a feature which is optionally supported by the hardware. your hardware doesnt support dma. thats not a problem, data access can happen in other ways too.

the piix driver does more things than just provide DMA. using the piix driver does not equal dma. specific ide drivers do not equal dma, although in most cases, they are required to get dma access going.

there are even some ide chipsets, which dont support dma, not even mwdma, but require their own specific IDE driver to even be able to use them. (they dont even work at all with the basic "generic ide" support)

the reason you cant get UDMA is because the IDE controller present on your motherboard doesnt support it. under normal circumstances, UDMA is implemented in hardware, but your hardware doesnt implement it.

there are a lot of PIIX controllers. some support UDMA, some dont. the piix driver in the kernel supports them all, regardless of their dma capabilities. you happen to have one of the few that dont do UDMA.

so yes, you shouldnt be able to get UDMA on the hardware that you have. piix isnt a specific chip, its a whole range of chips, with varying capabilities.

look at the kernel source here, it might make it clearer
http://sosdg.org/~coywolf/lxr/source/drivers/ide/pci/piix.c#L117
your device is the PCI_DEVICE_ID_INTEL_82371FB_1
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Bob P · Advocate Joined: 20 Oct 2004 Posts: 3374 Location: USA