Date: Mon, 23 Nov 2009 10:40:16 -0500 From: John Baldwin <jhb@freebsd.org> To: Scott Long <scottl@samsco.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Pyun YongHyeon <yongari@freebsd.org> Subject: Re: svn commit: r199670 - head/sys/dev/bge Message-ID: <200911231040.16597.jhb@freebsd.org> In-Reply-To: <2B4A2CD3-AA54-4030-9F39-3BF7DFC0434C@samsco.org> References: <200911222050.nAMKoRYh029141@svn.freebsd.org> <200911230838.20217.jhb@freebsd.org> <2B4A2CD3-AA54-4030-9F39-3BF7DFC0434C@samsco.org>
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On Monday 23 November 2009 10:06:08 am Scott Long wrote: > > On Nov 23, 2009, at 6:38 AM, John Baldwin wrote: > > > On Sunday 22 November 2009 6:48:18 pm Scott Long wrote: > >> By definition, PCIe can't transfer across 4GB boundaries. It's not a > >> bug specific to Broadcom. If you're loading dynamic buffers (i.e. > >> mbufs), setting an appropriate boundary value in the tag will allow > >> busdma to take care of this. If you're allocating static buffers, > >> busdma won't honor this. But what you've done here is best anyways; > >> control buffers that are going to be frequently transferred are best > >> kept in the lower 4GB of the address space. It simplifies PCIe > >> handling, and it's significantly faster on PCI/PCI-X. So I'd suggest > >> making this the rule rather than the exception in the driver. > > > > Should we enforce an implicit 4GB boundary in bus_dma then? Perhaps > > Host-PCI > > bridge drivers should create a tag with a 4GB boundary that devices > > inherit > > via bus_get_dma_tag(). For i386/PAE we might should always enforce > > a 4GB > > boundary as well? > > That was actually the point of creating bus_get_dma_tag(). I don't > recall how complete the back-end work of providing a inheritance tree > for bridges got. Note that having this wouldn't really solve Pyun's > problem, because boundaries aren't honored for static allocations. > There's been plenty of talk about multi-segment static allocations, > but I don't think that that's applicable to this either. As I said > before, it's best to restrict static allocations to the lower 4GB of > memory and not worry about the boundary at all. But having the proper > inheritance would still be nice. > > Another thing that I'd like to do is have an alternate for > bus_dma_tag_create() that takes the device_t of the device as the > first argument and figures out the parent tag and inheritance > automatically. That's a job for the mythical /sys/kern/subr_busdma.c. The inheritance thing will actually work as the default version of bus_dma_get_tag() is to just pass the request up the tree. As long as devices always use bus_dma_get_tag() when creating the tag they pass down to their children via their own bus_dma_get_tag() implementation all the various restrictions should be passed down the tree. -- John Baldwin
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