Chapter 21. Configuring Low-level Parameters
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## Chapter 21. Configuring Low-level Parameters

The NVIDIA resource manager recognizes several low-level configuration parameters that can be set using the sysctl driver interface before the X server is started. Normally you should not need to modify any of these parameters, but it is sometimes necessary or desirable to do so.

To view the current settings of these parameters, you need to issue this sysctl command (`nvidia.ko` needs to be loaded):

        % sysctl -a hw.nvidia.registry

To change any of the parameters, you need to pass the complete name of the OID followed by '=' and the new value, e.g.:

        % sysctl hw.nvidia.registry.EnableVia4x=1

It is possible to automate setting these parameters by adding them to the `/etc/sysctl.conf` file. See `man 5 sysctl.conf` for details.

The following parameters are recognized by `nvidia.ko`:

Resource Manager Parameters

`VideoMemoryTypeOverride`

  We normally detect memory type on TNT cards by scanning the embedded BIOS. Unfortunately, we've seen some cases where a TNT card has been flashed with the wrong BIOS. For example, an SDRAM based TNT has been flashed with an SGRAM BIOS, and therefore claims to be an SGRAM TNT. We've therefore provided an override here. Make sure to set the value toe the type of memory used on your card.

  Value | Meaning
  --- | ---
  1 | SDRAM
  2 | SGRAM

  Note that we can only do so much here. There are border cases where even this fails. For example, if 2 TNT cards are in the same system, one SGRAM, one SDRAM.

  This option is disabled by default, see below for information on how to enable it.

`EnableVia4x`

  We've had problems with some Via chipsets in 4x mode, we need force them back down to 2x mode. If you'd like to experiment with retaining 4x mode, you may try setting this value to 1 If that hangs the system, you're stuck with 2x mode; there's nothing we can do about it.

  Value | Meaning
  --- | ---
  0 | disable AGP 4x on Via chipsets (default)
  1 | enable AGP 4x on Via chipsets

`EnableALiAGP`

  Some ALi chipsets (ALi1541, ALi1647) are known to cause severe system stability problems with AGP enabled. To avoid lockups, we disable AGP on systems with these chipsets by default. It appears that updating the system BIOS and using recent versions of the kernel AGP Gart driver can make such systems much more stable. If you own a system with one of the aforementioned chipsets and had it working reasonably well previously, or if you want to experiment with BIOS and AGPGART revisions, you can re-enable AGP support by setting this option to 1.

  Value | Meaning
  --- | ---
  0 | disable AGP on Ali1541 and ALi1647 (default)
  1 | enable AGP on Ali1541 and ALi1647

`NvAGP`

  This options controls which AGP GART driver is used when no explicit request is made to change the default (X server).

  Value | Meaning
  --- | ---
  0 | disable AGP support
  1 | use the NVIDIA builtin driver (if possible)
  2 | use the kernel's AGPGART driver (if possible)
  3 | use any available driver (try 2, then 1)

  Note that the NVIDIA internal AGP GART driver will not be used if AGPGART was either statically linked into your kernel or built as a kernel module and loaded before the NVIDIA kernel module.

`ReqAGPRate`

  Normally, the driver will compare speed modes of the chipset and the card, picking the highest common rate. This key forces a maximum limit, to limit the driver to lower speeds. The driver will not attempt a speed beyond what the chipset and card claim they are capable of.

  Make sure you really know what you're doing before you enable this override. By default, AGP drivers will enable the fastest AGP rate your card and motherboard chipset are capable of. Then, in some cases, our driver will force this rate down to work around bugs in both our chipsets, and motherboard chipsets. Using this variable will override our bug fixes. This may be desirable in some cases, but not most. This is completely unsupported!

  This option expects a bitmask (7 = 1|2|3|4, 3=1|2, etc.)

  This option is disabled by default, see below for information on how to enable it.

`EnableAGPSBA`

  For stability reasons, the driver will not use Side Band Addressing even if both the host chipset and the AGP card support it. You may override this behavior with the following registry key. This is completely unsupported!

  Value | Meaning
  --- | ---
  0 | disable Side Band Addressing (default on x86, see below)
  1 | enable Side Band Addressing (if supported)

`EnableAGPFW`

  Similar to Side Band Addressing, Fast Writes are disabled by default. If you wish to enable them on systems that support them, you can do so with this registry key. Note that this may render your system unstable with many AGP chipsets. This is completely unsupported!

  Value | Meaning
  --- | ---
  0 | disable Fast Writes (default)
  1 | enable Fast Writes

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