[PATCH V3 6/8] arm: dma-mapping: Reset the device's dma_ops

On Wed, May 24, 2017 at 12:46:51AM +0300, Laurent Pinchart wrote:

Hi Russell,

On Tuesday 23 May 2017 18:53:19 Russell King - ARM Linux wrote:

quoted

On Tue, May 23, 2017 at 05:55:57PM +0100, Robin Murphy wrote:

quoted

On 23/05/17 17:25, Russell King - ARM Linux wrote:

quoted

So, I've come to apply this patch (since it's finally landed in the
patch system), and I'm not convinced that the commit message is really
up to scratch.

The current commit message looks like this:

"   ARM: 8674/1: dma-mapping: Reset the device's dma_ops
    arch_teardown_dma_ops() being the inverse of arch_setup_dma_ops(),
    dma_ops should be cleared in the teardown path. Otherwise this
    causes problem when the probe of device is retried after being
    deferred. The device's iommu structures are cleared after
    EPROBEDEFER error, but on the next try dma_ops will still be set to
    old value, which is not right."

It is obviously a fix, but a fix for which patch?  Looking at the
history, we have "arm: dma-mapping: Don't override dma_ops in
arch_setup_dma_ops()" which I would have guessed is the appropriate
one, but this post-dates your patch (it's very recent, v4.12-rc
recent.)

So, I need more description about the problem you were seeing when
you first proposed this patch.

How does leaving the dma_ops in place prior to "arm: dma-mapping:
Don't override dma_ops in arch_setup_dma_ops()" cause problems for
deferred probing?

What patch is your change trying to fix?  In other words, how far
back does this patch need to be backported?

In effect, it's fixing a latent inconsistency that's been present since
its introduction in 4bb25789ed28. However, that has clearly not proven
to be an issue in practice since then. With 09515ef5ddad we start
actually calling arch_teardown_dma_ops() in a manner that might leave
things partially initialised if anyone starts removing and reprobing
drivers, but so far that's still from code inspection[1] rather than
anyone hitting it.

Given that the changes which tickle it are fresh in -rc1 I'd say there's
no need to backport this, but at the same time it shouldn't do any
damage if you really want to.

Well, looking at this, I'm not convinced that much of it is correct.

1) set_dma_ops() is in arch_setup_dma_ops() but this patch adds
   the unsetting of the DMA ops inside arm_teardown_iommu_dma_ops()
   rather than arch_teardown_dma_ops().

   This doesn't strike me as being particularly symmetric.
   arm_teardown_iommu_dma_ops() is arm_setup_iommu_dma_ops()'s
   counterpart.

2) arch_setup_dma_ops(), the recent patch to add the existing dma_ops
   check, and Xen - Xen wants to override the DMA ops if in the
   initial domain.  It's not clear (at least to me) whether the recent
   patch adding the dma_ops check took account of this or not.

3) random places seem to fiddle with the dma_ops - notice that
   arm_iommu_detach_device() sets the dma_ops to NULL.

   In fact, I think moving __arm_iommu_detach_device() into
   arm_iommu_detach_device(), calling arm_iommu_detach_device(),
   and getting rid of the explicit set_dma_ops(, NULL) in this
   path would be a good first step.

4) I think arch_setup_dma_ops() is over-complex.

So, in summary, this code is a mess today, and that means it's not
obviously correct - which is bad.  This needs sorting.

We've reached the same conclusion independently, but I'll refrain from 
commenting on whether that's a good or bad thing :-)

I don't think this patch should be applied, as it could break Xen (and other 
platforms/drivers that set the DMA operations manually) by resetting DMA 
operations at device remove() time even if they have been set independently of 
arch_setup_dma_ops().

That will only occur if the dma ops have been overriden once the DMA
operations have been setup via arch_setup_dma_ops.  What saves it from
wholesale NULLing of the DMA operations is the check for a valid
dma_iommu_mapping structure in arm_teardown_iommu_dma_ops().  This only
exists when arm_setup_iommu_dma_ops() has attached a mapping to the
device.

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