diff options
| -rw-r--r-- | drivers/iommu/Kconfig | 26 | ||||
| -rw-r--r-- | drivers/iommu/dma-iommu.c | 2 |
2 files changed, 27 insertions, 1 deletions
diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig index c79a0df090c0..5a225b48dd00 100644 --- a/drivers/iommu/Kconfig +++ b/drivers/iommu/Kconfig @@ -144,6 +144,32 @@ config IOMMU_DMA select IRQ_MSI_IOMMU select NEED_SG_DMA_LENGTH +config IOMMU_DMA_PCI_SAC + bool "Enable 64-bit legacy PCI optimisation by default" + depends on IOMMU_DMA + help + Enable by default an IOMMU optimisation for 64-bit legacy PCI devices, + wherein the DMA API layer will always first try to allocate a 32-bit + DMA address suitable for a single address cycle, before falling back + to allocating from the device's full usable address range. If your + system has 64-bit legacy PCI devices in 32-bit slots where using dual + address cycles reduces DMA throughput significantly, this may be + beneficial to overall performance. + + If you have a modern PCI Express based system, this feature mostly just + represents extra overhead in the allocation path for no practical + benefit, and it should usually be preferable to say "n" here. + + However, beware that this feature has also historically papered over + bugs where the IOMMU address width and/or device DMA mask is not set + correctly. If device DMA problems and IOMMU faults start occurring + after disabling this option, it is almost certainly indicative of a + latent driver or firmware/BIOS bug, which would previously have only + manifested with several gigabytes worth of concurrent DMA mappings. + + If this option is not set, the feature can still be re-enabled at + boot time with the "iommu.forcedac=0" command-line argument. + # Shared Virtual Addressing config IOMMU_SVA bool diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c index f90251572a5d..9f9d9ba7f376 100644 --- a/drivers/iommu/dma-iommu.c +++ b/drivers/iommu/dma-iommu.c @@ -67,7 +67,7 @@ struct iommu_dma_cookie { }; static DEFINE_STATIC_KEY_FALSE(iommu_deferred_attach_enabled); -bool iommu_dma_forcedac __read_mostly; +bool iommu_dma_forcedac __read_mostly = !IS_ENABLED(CONFIG_IOMMU_DMA_PCI_SAC); static int __init iommu_dma_forcedac_setup(char *str) { |