Re: Can Linux live without DMA zone?

[Stephen Hemminger <shemminger@osdl.org>]

On Thu, 2 Nov 2006 15:17:15 -0800
Jun Sun <jsun@junsun.net> wrote:

> On Thu, Nov 02, 2006 at 11:19:05PM +0100, Arjan van de Ven wrote:
> > On Thu, 2006-11-02 at 16:26 -0500, Phillip Susi wrote:
> > > Arjan van de Ven wrote:
> > > > that's for the 32 bit boundary. THe problem is that there are 31, 30, 28
> > > > and 26 bit devices as well, and those are in more trouble, and will
> > > > eventually fall back to GFP_DMA (inside the x86 PCI code; the driver
> > > > just uses the pci dma allocation routines) if they can't get suitable
> > > > memory otherwise....
> > > > 
> > > > It's all nice in theory. But then there is the reality that not all
> > > > devices are nice pci device that implement the entire spec;)
> > > > 
> > > 
> > > Right, but doesn't the bounce/allocation routine take as a parameter the 
> > > limit that the device can handle?  If the device can handle 28 bit 
> > > addresses, then the kernel should not limit it to only 24 bits.
> > 
> > you're right in theory, but the kernel only has a few pools of memory
> > available, but not at every bit boundary. there is a 32 bit pool
> > (GFP_DMA32) on some, a 30-ish bit pool (GFP_KERNEL) on others, and a 24
> > bit pool (GFP_DMA) with basically nothing inbetween.
> >
> 
> Perhaps a better solution is to 
> 
> 1. get rid of DMA zone
> 
> 2. have another alloc funciton (e.g., kmalloc_range()) which takes an
>    extra pair of parameters to indicate the desired range for the
>    allocated memory.  Most DMA buffers are allocated during start-up.
>    So the alloc operations should generally be successful.
> 

Network devices don't allocate buffer until they are brought up.
By then a lot of memory allocation has happened.  You could add an
interface that allows a device to say:
	kmalloc_range_intent(unsigned long mask, unsigned count, unsigned size)
to cause reservation before use.

-- 
Stephen Hemminger <shemminger@osdl.org>