netdev - Re: network devices don't handle pci_dma_mapping

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Date:	Wed, 6 Dec 2006 16:58:35 -0800
From:	Stephen Hemminger <shemminger@...l.org>
To:	David Miller <davem@...emloft.net>
Cc:	muli@...ibm.com, jeff@...zik.org, amitkale@...xen.com,
	netdev@...r.kernel.org, brazilnut@...ibm.com,
	netxenproj@...syssoft.com, rob@...xen.com, romieu@...zoreil.com,
	sanjeev@...xen.com, wendyx@...ibm.com
Subject: Re: network devices don't handle pci_dma_mapping_error()'s

On Wed, 06 Dec 2006 16:54:18 -0800 (PST)
David Miller <davem@...emloft.net> wrote:

> From: Stephen Hemminger <shemminger@...l.org>
> Date: Wed, 6 Dec 2006 10:16:44 -0800
> 
> > I think it is really only an issue for drivers that turn on HIGH_DMA
> > and have limited mask values. The majority of drivers either only handle
> > 32 bit (!HIGH_DMA) or do full 64 bit mapping. I don't know the details
> > of how we manage IOMMU, but doesn't mapping always work for those drivers.
> > 
> > That just leaves devices with odd size mask values that need to be
> > handle mapping errors.
> 
> Not true.
> 
> On platforms such as sparc64 the IOMMU is used for all DMA mappings,
> no matter what, because only IOMMU based mappings can do prefetching
> and write-combining in the PCI controller.
> 
> The problem with just silently dropping packets that can't get DMA
> mapped is that you're going to drop a very large sequence of these
> while the IOMMU is out of space, and that to me looks like a bad
> quality of implementation decision.
> 
> The IOMMU layer really needs a way to callback the driver to tell it
> when space is available, or something similar.
> 
> FWIW, Solaris handles this by blocking when the IOMMU is out of space
> since under Solaris even interrupt contexts can block (via interrupt
> threads).
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The more robust way would be to stop the queue (like flow control)
and return busy. You would need a timer though to handle the case
where some disk i/o stole all the mappings and then network device flow
blocked.
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