[prev in list] [next in list] [prev in thread] [next in thread] 

List:       linux-arm-kernel
Subject:    Re: DMA memory ?
From:       Manfred Gruber <gruber.m () utanet ! at>
Date:       2005-05-02 11:05:34
Message-ID: 200505021305.34742.gruber.m () utanet ! at
[Download RAW message or body]

hi !

sorry that I write back so late but i was busy.
can you explain me if i had understand right what you have written.

Am Freitag, 22. April 2005 20:52 schrieb Russell King - ARM Linux:
> On Thu, Apr 21, 2005 at 06:02:17PM +0200, Manfred Gruber wrote:
> > In  linux/arch/arm/mm/consistent.c is defined :
> > #define CONSISTENT_BASE	(0xffc00000)
> > #define CONSISTENT_END		(0xffe00000)
> >
> > Which allowes my to alloc 2MB DMA memory. When I have now a big
> > framebuffer driver, say 800x600 with 32 bit i need 1,8MB DMA Memory. When
> > I now want to add sounddriver DMA memory it fails because there is
> > nothing more there.
> >
> > Is it allowed and correct to change this settings to get more DMA memory?
> > Or do I get troubles on other places ?
>
> 2MB isn't some random number - it's chosen because that's how much one
> page table is able to map.  The DMA code only knows how to handle one
> page table worth of virtual pages, so by increasing it, you will end
> up writing off the end of that page table - scribbling into the memory
> following.

so dma_alloc_coherent can now only allocate 2MB for all devices (framebuffer, 
net, sound,...) because dma code only handles one page of.
 
> My preferred solution to this would be to completely split the kernel
> page table handling code from the user space handling code, so we don't
> have to have the overhead of managing the additional user space bits
> for kernel pages.  We can then allocate N page(s) which will cover
> N*4MB of virtual space.

that sound not trivial for me, correct ?

is there another sollution to allocate more DMA memory?

regards manfred 


-------------------------------------------------------------------
Subscription options: http://lists.arm.linux.org.uk/mailman/listinfo/linux-arm-kernel
FAQ:       http://www.arm.linux.org.uk/armlinux/mlfaq.php
Etiquette: http://www.arm.linux.org.uk/armlinux/mletiquette.php
[prev in list] [next in list] [prev in thread] [next in thread]