On 30/04/18 12:02, Thierry Reding wrote:
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@xxxxxxxxxx>
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).
It sounds like things are getting a bit backwards here: iommu-dma should
have nothing to do with this, since if you've explicitly attached the
device to your own IOMMU domain then you're already bypassing everything
it knows about and has control over. Arch code calling into iommu-dma to
do something which makes arch code not use iommu-dma makes very little
sense.
AFAICS the only aspect of arm_iommu_detach_device() which actually
matters in this case is the set_dma_ops() bit, so what we're really
after is a generic way for drivers to say "Hey, I actually have my own
MMU (or want to control the one you already know about) so please give
me direct DMA ops", which the arch code handles as appropriate (maybe
it's also allowed to fail in cases like swiotlb=force or when there is
RAM beyond the device's DMA mask).
Robin.
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