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. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
