On Tue, Apr 24, 2018 at 11:43:35PM -0700, Christoph Hellwig wrote: > On Wed, Apr 25, 2018 at 08:23:15AM +0200, Daniel Vetter wrote: > > For more fun: > > > > https://www.spinics.net/lists/dri-devel/msg173630.html > > > > Yeah, sometimes we want to disable the iommu because the on-gpu > > pagetables are faster ... > > I am not on this list, but remote NAK from here. This needs an > API from the iommu/dma-mapping code. Drivers have no business poking > into these details. The interfaces that the above patch uses are all EXPORT_SYMBOL_GPL, which is rather misleading if they are not meant to be used by drivers directly. > Thierry, please resend this with at least the iommu list and > linux-arm-kernel in Cc to have a proper discussion on the right API. I'm certainly open to help with finding a correct solution, but the patch above was purposefully terse because this is something that I hope we can get backported to v4.16 to unbreak Nouveau. Coordinating such a backport between ARM and DRM trees does not sound like something that would help getting this fixed in v4.16. The fundamental issue here is that the DMA/IOMMU integration is something that has caused a number of surprising regressions in the past because it tends to sneak in unexpectedly. For example the current regression shows up only if CONFIG_ARM_DMA_USE_IOMMU=y because the DMA API will then transparently create a second mapping and mess things up. Everything works fine if that option is disabled. This is ultimately why we didn't notice, since we don't enable that option by default. I do have a patch that I plan to apply to the Tegra tree that will always enable CONFIG_ARM_DMA_USE_IOMMU=y on Tegra to avoid any such surprises in the future, but I can obviously only apply that once the above patch is applied to Nouveau, otherwise we'll break Nouveau unconditionally. Granted, this issue could've been caught with a little more testing, but in retrospect I think it would've been a lot better if ARM_DMA_USE_IOMMU was just enabled unconditionally if it has side-effects that platforms don't opt in to but have to explicitly opt out of. Thierry
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