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iommu/dma: Add config for PCI SAC address trick
For devices stuck behind a conventional PCI bus, saving extra cycles at 33MHz is probably fairly significant. However since native PCI Express is now the norm for high-performance devices, the optimisation to always prefer 32-bit addresses for the sake of avoiding DAC is starting to look rather anachronistic. Technically 32-bit addresses do have shorter TLPs on PCIe, but unless the device is saturating its link bandwidth with small transfers it seems unlikely that the difference is appreciable. What definitely is appreciable, however, is that the IOVA allocator doesn't behave all that well once the 32-bit space starts getting full. As DMA working sets get bigger, this optimisation increasingly backfires and adds considerable overhead to the dma_map path for use-cases like high-bandwidth networking. We've increasingly bandaged the allocator in attempts to mitigate this, but it remains fundamentally at odds with other valid requirements to try as hard as possible to satisfy a request within the given limit; what we really need is to just avoid this odd notion of a speculative allocation when it isn't beneficial anyway. Unfortunately that's where things get awkward... Having been present on x86 for 15 years or so now, it turns out there are systems which fail to properly define the upper limit of usable IOVA space for certain devices and this trick was the only thing letting them work OK. I had a similar ulterior motive for a couple of early arm64 systems when originally adding it to iommu-dma, but those really should be fixed with proper firmware bindings by now. Let's be brave and default it to off in the hope that CI systems and developers will find and fix those bugs, but expect that desktop-focused distro configs are likely to want to turn it back on for maximum compatibility. Signed-off-by:Robin Murphy <robin.murphy@arm.com> Reviewed-by: