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

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Tue, May 23, 2017 at 05:55:57PM +0100, Robin Murphy wrote:
> On 23/05/17 17:25, Russell King - ARM Linux wrote:
> > 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.

-- 
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