linux-kernel - Re: Can Linux live without DMA zone?

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Message-ID: <20061102231715.GA10902@srv.junsun.net>
Date:	Thu, 2 Nov 2006 15:17:15 -0800
From:	Jun Sun <jsun@...sun.net>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Phillip Susi <psusi@....rr.com>, linux-kernel@...r.kernel.org
Subject: Re: Can Linux live without DMA zone?

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.

3. convert drivers over to use the new function.

Cheers.

Jun
   are allocated at start-up time.
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