Re: Can I get 200M contiguous physical memory?

[linux-os]

On Thu, 10 Mar 2005, Nate Edel wrote:

> From: "Arjan van de Ven" <arjan@xxxxxxxxxxxxx>
> To: "Jason Luo" <abcd.bpmf@xxxxxxxxx>
> > > A data acquisition card. In DMA mode, the card need 200M contiguous
> > > memory for DMA.
> >
> > (or want to reserve memory at the boot commandline and then do really
> > really evil hacks)
>
> Such as booting the machine with "mem=(real memory - 200)M" and then just 
> doing an ioremap of the top 200M of memory.
>
> It's not the most elegant way of doing things given that it requires user 
> intervention at boot time, but I'm not sure it counts as a "really evil 
> hack."  Code-wise it's very simple - there's sample code in a couple of the 
> in-RAM MTD(*) drivers you can use as a model. I'm not sure if this method 
> will translate easily to non-x86 platforms if that's an issue.
>
> (* /drivers/mtd/devices/slram.c and /drivers/mtd/devices/mtdram.c ; I'm not 
> sure which of these is more up to date.) 
> -

It doesn't require user-intervention either you can put a "mem="
statement in your boot configuration. Here's some GRUB stuff:

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You do not have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /, eg.
#          root (hd0,0)
#          kernel /boot/vmlinuz-version ro root=/dev/hda1
#          initrd /boot/initrd-version.img
#boot=/dev/hda
default=1
timeout=10
splashimage=(hd0,0)/boot/grub/splash.xpm.gz
title Fedora Core (2.6.11)
	root (hd0,0)
	kernel /boot/vmlinuz-2.6.11 ro root=LABEL=/ mem=768m rhgb
	initrd /boot/initrd-2.6.11.img

[SNIPPED...]

Also, You should NEVER require 200 MB of contiguous memory! The Bus-
Master controllers (all of them) provide for scatter-lists. You just
need to know how to program them. You just get the bus address of
every page in an allocated buffer. You just put those addresses in
the linked-list.

Even the old ISA DMA controller can only do 128k (64k words) at
a time. Therefore, you only need 128k of physical memory locked
down at any one time.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.10 on an i686 machine (5537.79 BogoMips).
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