From: David Mosberger <davidm@napali.hpl.hp.com>
To: hch@infradead.org, James.Bottomley@SteelEye.com
Cc: davem@redhat.com, axboe@suse.de, linux-kernel@vger.kernel.org
Subject: Re: problem with blk_queue_bounce_limit()
Date: Tue, 10 Jun 2003 13:24:06 -0700 [thread overview]
Message-ID: <16102.15974.597441.186748@napali.hpl.hp.com> (raw)
In-Reply-To: <16097.37454.827982.278024@napali.hpl.hp.com>
Christoph,
You wrote:
> imo PCI_DMA_BUS_IS_PHYS should be a propert of each struct device
> because a machine might have a iommu for one bus type but not
> another, e.g.
> dma_is_phys(dev);
As pointed out by DaveM, this isn't sufficient for the block layer,
which needs to know the page size of the I/O MMU so it can make
merging decisions about physically discontiguous buffers.
I think we also need:
/*
* Returns a mask of bits which need to be 0 in order for
* the DMA-mapping interface to be able to remap a buffer.
* DMA-mapping implementations for real (hardware) I/O MMUs
* will want to return (iommu_page_size - 1) here, if they
* support such remapping. DMA-mapping implementations which
* do not support remapping must return a mask of all 1s.
*/
unsigned long dma_merge_mask(dev)
Then you can replace:
BIO_VMERGE_BOUNDARY => (dma_merge_mask(dev) + 1)
Of course, this doesn't work literally, because we don't have a device
pointer handy in the bio code. Instead, it would probably make the
most sense to add a "iommu_merge_mask" member to "struct
request_queue" and then do something along the lines of:
#define BIO_VMERGE_BOUNDARY(q) ((q)->iommu_merge_mask + 1)
Note 1: the "+ 1" will get optimized away because the only way
BIO_VMERGE_BOUNDARY() is used is in BIOVEC_VIRT_MERGEABLE, which
really needs a mask anyhow; this could be cleaned up of course, but
that's a separate issue.
Note 2: dma_merge_mask() cannot be used to replace dma_is_phys() (as
much as I'd like that), because they issue of (virtual) remapping is
really quite distinct from whether a (hardware) I/O MMU is present
(not to mention the _other_ reason that a bus may not be "physical").
Note 3: I'm not comfortable hacking the bio code, so if someone would
like to prototype this, by all means go ahead... ;-)
Thanks,
--david
next prev parent reply other threads:[~2003-06-10 20:12 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-06-05 6:42 problem with blk_queue_bounce_limit() David Mosberger
2003-06-05 7:20 ` David S. Miller
2003-06-06 6:42 ` David Mosberger
2003-06-06 6:45 ` David S. Miller
2003-06-06 6:54 ` David Mosberger
2003-06-06 7:08 ` David S. Miller
2003-06-06 6:52 ` David S. Miller
2003-06-06 7:19 ` David Mosberger
2003-06-06 7:32 ` David S. Miller
2003-06-06 7:44 ` Christoph Hellwig
2003-06-06 7:43 ` David S. Miller
2003-06-06 7:51 ` Christoph Hellwig
2003-06-06 20:13 ` David Mosberger
2003-06-07 6:44 ` David S. Miller
2003-06-07 7:05 ` David Mosberger
2003-06-07 7:11 ` David S. Miller
2003-06-10 20:01 ` David Mosberger
2003-06-12 6:47 ` David S. Miller
2003-06-15 7:06 ` Anton Blanchard
2003-06-15 7:11 ` David S. Miller
2003-06-15 8:04 ` Anton Blanchard
2003-06-15 8:18 ` David S. Miller
2003-06-07 7:20 ` David Mosberger
2003-06-07 7:19 ` David S. Miller
2003-06-07 9:44 ` Russell King
2003-06-07 9:47 ` David S. Miller
2003-06-07 13:23 ` Christoph Hellwig
2003-06-10 20:24 ` David Mosberger [this message]
2003-06-06 18:06 James Bottomley
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