linux-kernel - Re: [openib-general] Suggestions for how to remove bus_to

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Message-Id: <1152916027.4572.391.camel@brick.pathscale.com>
Date:	Fri, 14 Jul 2006 15:27:07 -0700
From:	Ralph Campbell <ralphc@...hscale.com>
To:	Muli Ben-Yehuda <muli@...ibm.com>
Cc:	David Miller <davem@...emloft.net>, rdreier@...co.com,
	linux-kernel@...r.kernel.org, openib-general@...nib.org
Subject: Re: [openib-general] Suggestions for how to remove bus_to_virt()

On Thu, 2006-07-13 at 08:46 +0300, Muli Ben-Yehuda wrote:
> On Wed, Jul 12, 2006 at 05:40:13PM -0700, David Miller wrote:
> > From: Roland Dreier <rdreier@...co.com>
> > Date: Wed, 12 Jul 2006 17:11:26 -0700
> > 
> > > A cleaner solution would be to make the dma_ API really use the device
> > > it's passed anyway, and allow drivers to override the standard PCI
> > > stuff nicely.  But that would be major surgery, I guess.
> > 
> > Clean but expensive, you should not force the rest of the kernel
> > to eat the cost of something you want to do when it's totally
> > unnecessary for most other users.
> > 
> > For example, x86 never needs to do anything other than a direct
> > virt_to_phys translation to produce a DMA address, no matter what
> > bus the device is on.  It's a single simple integer adjustment
> > that can be done inline in about 2 or 3 instructions at most.
> 
> It's possible that even x86 will support multiple IOMMUs in the future
> - for example, the Calgary IOMMU support we recently added to x86-64
> could be modified to work on plain x86 as well.
> 
> I like the idea of a per-device DMA-API implementation, but only if it
> can be done in a way that is zero cost to the majority of the users of
> the API. We already have dynamic dma_ops on x86-64 to support nommu,
> swiotlb, gart and Calgary cleanly, extending it to use a per-device
> dma-ops isn't too difficult.
> 
> Cheers,
> Muli

A per-device DMA-API would solve my problem.
It would be a fairly invasive changeset though.
The basic idea would be to add a struct dma_mapping_ops *
to struct device and change all the inline dma_* routines
to something like:

static inline dma_addr_t
dma_map_single(struct device *hwdev, void *ptr, size_t size,
               int direction)
{
        BUG_ON(!valid_dma_direction(direction));
        return hwdev->dma_ops ? 
                hwdev->dma_ops->map_single(hwdev, ptr, size, direction) :
                dma_ops->map_single(hwdev, ptr, size, direction);
}

Note that the current design only supports one IOMMU type in a system.
This could support multiple IOMMU types at the same time.

Another possibility is to only do this for the infiniband subsystem.
The idea would be to replace calls to dma_* with ib_dma_* which
would be defined as above.

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