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Everybody considers today's graphic cards to be meant only for gamers. After all, why would one need a fast GPU in a server? Today's graphic cards contain a lot of very fast RAM, typically between 64 and 512 MB. With Linux, it's possible to use it as swap space, or even as RAM disk!

Preparing the kernel

To use the graphic (and possibly any other) card's memory, it needs to be mapped by the kernel. By default only the system RAM is considered as usable RAM, to access memory from the PCI address space one needs a driver which registers them. In 2.6 kernels this driver is shipped: Memory Technology Device (MTD) support. Bring up menuconfig, then:

Linux Kernel Configuration: Enable MTD kernel features
Device Drivers  --->
  Memory Technology Devices (MTD)  --->
    Memory Technology Device (MTD) support
     [* or M]Direct char device access to MTD devices
     [* or M]Caching block device access to MTD devices
     Self-contained MTD device drivers  --->
      [* or M]Uncached system RAM
      [* or M]Physical system RAM

You can compile them as modules or built-in drivers, it works either way.

Note: "Uncached system RAM" is the slram driver, the "Physical system RAM" is the phram. They can co-exist (but can't use the same memory region), and phram is the successor of slram, I'm just too lazy to test it. If you have free time, please try it and update this page.

Finding the video memory

Now let's search for the video memory in the PCI space. The easiest way is to use lspci:

Code: working with lspci
# lspci -vvv

0000:02:00.0 VGA compatible controller: ATI Technologies Inc R300 AD [Radeon 9500 Pro] (prog-if 00 [VGA])
        Subsystem: PC Partner Limited: Unknown device 7c07
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping+ SERR- FastB2B-
        Status: Cap+ 66Mhz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 32 (2000ns min), cache line size 08
        Interrupt: pin A routed to IRQ 255
        Region 0: Memory at d8000000 (32-bit, prefetchable) [size=128M]
        Region 1: I/O ports at b000 [size=256]
        Region 2: Memory at e9000000 (32-bit, non-prefetchable) [size=64K]
        Expansion ROM at e8000000 [disabled] [size=128K]
        Capabilities: [58] AGP version 3.0
                Status: RQ=256 Iso- ArqSz=0 Cal=0 SBA+ ITACoh- GART64- HTrans- 64bit- FW+ AGP3+ Rate=x4,x8
                Command: RQ=1 ArqSz=0 Cal=0 SBA+ AGP- GART64- 64bit- FW- Rate=<none>
        Capabilities: [50] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 PME-Enable- DSel=0 DScale=0 PME-

What we need is the prefetchable memory region, Region 0. The start of the memory space is 0xd8000000, a 32 bit hex value.

Warning: The above is an example. On your computer the memory region's start value could be different. Always check!

Mapping the memory

To actually access the memory from PCI space, you need to tell the MTD driver where it starts and where it ends. Don't worry, it's easy to calculate the ending address! Just decide how much memory you want to use. However, it's wise to leave some for the video card too, to let its basic functionalities work. Now, if you compiled the SLRAM driver as module, run:

Code: Loading SLRAM module
# Region 0 starts at 0xd8000000, leave 4 MB ram for the VGA, so add 4*1024*1024 to it
# which is 0x00400000 in hex, 32 bit. This means: 0xd8400000 will be the starting position.
# Now, to use the other 124 MB, the easiest way is to tell the offset value.
# The third parameter's leading + means that we are telling the offset,
# not the actual region's ending address. 124 MB is 0x7c00000 in hex, 32 bit.

modprobe slram map=VRAM,0xd8400000,+0x7c00000
Code: Loading PHRAM module
# Loading with PHRAM module is very similar to SLRAM module, except that it uses a "length"
# parameter by default rather than an "end offset".

# This does the same thing as the SLRAM module above
modprobe phram phram=VRAM,0xd8400000,0x7c00000

# This is also the same (and easier to read)
modprobe phram phram=VRAM,0xd8400000,124Mi

or if you compiled it as built-in, change your kernel command line to contain this:

Code: Adding SLRAM configuration to the kernel command line
# Now the first parameter is the device which will be mapped. 
# Other parameters share the same meaning shown above, in the module version.
# Please reboot your computer, to take it effect.


What you should see in dmesg is:

Code: Kernel output
slram: devname = mtd0
slram: devstart = 0xd8400000
slram: devlength = +0x7c00000
slram: devname=mtd0, devstart=0xd8400000, devlength=0x7c00000
slram: Registered device mtd0 from 3543040KiB to 3670016KiB
slram: Mapped from 0xe0880000 to 0xe8480000
Code: Also, to be sure, check /proc/mtd
dev:    size   erasesize  name
mtd0: 07c00000 00004000 "mtd0"

Creating the mtd devices

After you have the support in the kernel and all the required data, you need way to access the memory. Time for creating the devices!

If you have udev just modprobe mtdchar and modprobe mtdblock and hald will create device files for you automatically.

Code: The mknod magic
# Create the character device. MTD will use and map this, so it's not used by
# userspace programs but is vital for MTD to function.
# If "Direct char device access to MTD devices" was compiled as module, run modprobe first: modprobe mtdchar

mknod /dev/mtd0 c 90 0

# Create the block device. This will be used. Requires the "Caching block device access to
# MTD devices" kernel feautre, if compiled as module, first run: modprobe mtdblock

mknod /dev/mtdblock0 b 31 0

Using it

Now, you can either use it as a RAM Disk, or as swap. Either way, you need to create a FS on it, I'll go with the swap way.

Code: Creating swap
mkswap /dev/mtdblock0
swapon /dev/mtdblock0

This will create the swap structure in the video card's RAM, and also activate it as system swap. To specify the priority (which swap source will be used first in the swap list), use swapon's switch, -p X where X is a number between 0..32767, the higher the number, the higher the priority.

Code: Specified priority for swap

swapon /dev/mtdblock0 -p 10

Without specifying priority, the next lowest will be used. Also, this can be added to the fstab, so you won't need to manually add this after every boot:

Code: /etc/fstab
# <fs>                          <mountpoint>    <type>          <opts>                  <dump/pass>
/dev/mtdblock0                  none            swap            sw,pri=10

In the options field, the pri= is the same as swapon's -p switch. Please read the troubleshooting section, before closing this page.

Be warned

It's nice to have fast swap or RAM disk on your home computer but be warned, if a binary driver is loaded for X, it may freeze the whole system or create graphical glitches. Usually there is no way to tell the driver how much memory could be used, so it won't know the upper limit. However, the VESA driver can be used because it provides the possibility to set the video RAM size.

So, Direct Rendering or fast swap. Your choice.

Unlike motherboard RAM and hard drives, there aren't any known video cards that have ECC memory. This may not be a big deal for graphics rendering, but you definitely don't want to put critical data in it or use this feature on servers.


Credits goes to

This article was plagi^W inspired by the article written by Michal Schulz, available here: [1] and was summarized by me.

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Last modified: Fri, 05 Sep 2008 15:27:00 +1000 Hits: 38,139

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