PCI specs permit zero length reads (ZLR) even if the mapping for that region is write only. Support for this feature is indicated by the presence of a bit in the DMAR capability. If a particular DMAR does not support this capability we map write-only regions as read-write. This option can also provides a workaround for some drivers that request a write-only mapping when they really should request a read-write. (We ran into one such case in eepro100.c in handling rx_ring_dma) Signed-off-by: Ashok Raj Signed-off-by: Shaohua Li ---------------------------------- drivers/pci/intel-iommu.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) Index: linux-2.6.21-rc5/drivers/pci/intel-iommu.c =================================================================== --- linux-2.6.21-rc5.orig/drivers/pci/intel-iommu.c 2007-04-09 03:05:25.000000000 -0700 +++ linux-2.6.21-rc5/drivers/pci/intel-iommu.c 2007-04-09 03:05:32.000000000 -0700 @@ -84,7 +84,7 @@ struct sys_device sysdev; }; -static int dmar_disabled; +static int dmar_disabled, dmar_force_rw; static char *get_fault_reason(u8 fault_reason) { @@ -102,6 +102,9 @@ if (!strncmp(str, "off", 3)) { dmar_disabled = 1; printk(KERN_INFO"Intel-IOMMU: disabled\n"); + } else if (!strncmp(str, "forcerw", 7)) { + dmar_force_rw = 1; + printk(KERN_INFO"Intel-IOMMU: force R/W for W/O mapping\n"); } str += strcspn(str, ","); while (*str == ',') @@ -1668,7 +1671,12 @@ goto error; } - if (dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL) + /* + * Check if DMAR supports zero-length reads on write only + * mappings.. + */ + if (dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL || \ + !cap_zlr(domain->iommu->cap) || dmar_force_rw) prot |= DMA_PTE_READ; if (dir == DMA_FROM_DEVICE || dir == DMA_BIDIRECTIONAL) prot |= DMA_PTE_WRITE; Index: linux-2.6.21-rc5/include/linux/intel-iommu.h =================================================================== --- linux-2.6.21-rc5.orig/include/linux/intel-iommu.h 2007-04-09 03:05:25.000000000 -0700 +++ linux-2.6.21-rc5/include/linux/intel-iommu.h 2007-04-09 03:05:32.000000000 -0700 @@ -79,6 +79,7 @@ #define cap_max_fault_reg_offset(c) \ (cap_fault_reg_offset(c) + cap_num_fault_regs(c) * 16) +#define cap_zlr(c) (((c) >> 22) & 1) #define cap_isoch(c) (((c) >> 23) & 1) #define cap_mgaw(c) ((((c) >> 16) & 0x3f) + 1) #define cap_sagaw(c) (((c) >> 8) & 0x1f) -- - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/