From: "Adam J. Richter" <adam@yggdrasil.com>
Date: Thu, 2 Jan 2003 09:04:57 -0800
Message-Id: <200301021704.JAA00852@adam.yggdrasil.com>
To: James.Bottomley@steeleye.com
Subject: Re: [PATCH] generic device DMA (dma_pool update)
Cc: akpm@digeo.com, david-b@pacbell.net, linux-kernel@vger.kernel.org
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2241
Lines: 41

James Bottomley wrote:
>adam@yggdrasil.com said:
>> 	Let me clarify or revise my request.  By "show me or invent an
>> example" I mean describe a case where this would be used, as in
>> specific hardware devices that Linux has trouble supporting right now,
>> or specific programs that can't be run efficiently under Linux, etc.
>> What device would need to do this kind of allocation?  Why haven't I
>> seen requests for this from people working on real device drivers?
>> Where is this going to make the kernel smaller, more reliable, faster,
>> more maintainable, able to make a computer do something it could do
>> before under Linux, etc.?

>I'm not really the right person to be answering this.  For any transfer you 
>set up (which encompasses all of the SCSI stuff bar target mode and AENs) you 
>should have all the resources ready and waiting in the interrupt, and so never 
>require an in_interrupt allocation.

>However, for unsolicited transfer requests---the best example I can think of 
>would be incoming network packets---it does make sense:  You allocate with 
>GFP_ATOMIC, if the kernel can fulfil the request, fine; if not, you drop the 
>packet on the floor.  Now, whether there's an unsolicited transfer that's 
>going to require coherent memory, that I can't say.  It does seem to be 
>possible, though.

	When a network device driver receives a packet, it gives the
network packet that it pre-allocated to the higher layers with
netif_rx() and then allocates a net packet with dev_alloc_skb(), but
that is non-consistent "streaming" memory.  The consistent memory is
general for the DMA gather-scatter stub(s), which is (are) generally
reused since the receive for the packet that arrived has been
completed.

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