Nouveau - Apr 2018 - [PATCH v2 2/5] dma-mapping: Introduce dma_iommu_detach_device() API

On Wed, Apr 25, 2018 at 12:10:48PM +0200, Thierry Reding
wrote:
> From: Thierry Reding <treding at nvidia.com>
> 
> The dma_iommu_detach_device() API can be used by drivers to forcibly
> detach a device from an IOMMU that architecture code might have attached
> to. This is useful for drivers that need explicit control over the IOMMU
> using the IOMMU API directly.
Given that no one else implements it making it a generic API seems
rather confusing.  For now I'd rename it to
arm_dma_iommu_detach_device() and only implement it in arm.

Once I've got the dma mapping implementations consolidated to a small
enough number we could think about something like a device quirk that
tells the architecture to simply never even attach the iommu dma ops
to start with.

On Wed, Apr 25, 2018 at 08:19:34AM -0700, Christoph Hellwig
wrote:
> On Wed, Apr 25, 2018 at 12:10:48PM +0200, Thierry Reding wrote:
> > From: Thierry Reding <treding at nvidia.com>
> > 
> > The dma_iommu_detach_device() API can be used by drivers to forcibly
> > detach a device from an IOMMU that architecture code might have
attached
> > to. This is useful for drivers that need explicit control over the
IOMMU
> > using the IOMMU API directly.
> 
> Given that no one else implements it making it a generic API seems
> rather confusing.  For now I'd rename it to
> arm_dma_iommu_detach_device() and only implement it in arm.
That'd be suboptimal because this code is used on both 32-bit and 64-bit
ARM. If we make the function 32-bit ARM specific then the driver code
would need to use an #ifdef to make sure compilation doesn't break on
64-bit ARM.

Thierry
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On Thu, Apr 26, 2018 at 02:11:36PM +0200, Thierry Reding
wrote:
> On Wed, Apr 25, 2018 at 08:19:34AM -0700, Christoph Hellwig wrote:
> > On Wed, Apr 25, 2018 at 12:10:48PM +0200, Thierry Reding wrote:
> > > From: Thierry Reding <treding at nvidia.com>
> > > 
> > > The dma_iommu_detach_device() API can be used by drivers to
forcibly
> > > detach a device from an IOMMU that architecture code might have
attached
> > > to. This is useful for drivers that need explicit control over
the IOMMU
> > > using the IOMMU API directly.
> > 
> > Given that no one else implements it making it a generic API seems
> > rather confusing.  For now I'd rename it to
> > arm_dma_iommu_detach_device() and only implement it in arm.
> 
> That'd be suboptimal because this code is used on both 32-bit and
64-bit
> ARM. If we make the function 32-bit ARM specific then the driver code
> would need to use an #ifdef to make sure compilation doesn't break on
> 64-bit ARM.
Do you still want me to make this ARM specific? While I haven't
encountered this issue on 64-bit ARM yet, I think it would happen there
as well, under the right circumstances. I could take a shot at
implementing the equivalent there (which means essentially implementing
it for drivers/iommu/dma-iommu.c and calling that from 64-bit ARM code).

Thierry
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