[Intel IOMMU][patch 4/8] Supporting Zero Length Reads in Intel IOMMU.

Ashok Raj
2007-04-24T15:08:28Z

[-- Attachment #1: rw-workaround.patch --]
[-- Type: text/plain, Size: 2542 bytes --]

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 <ashok.raj@intel.com>
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
----------------------------------
drivers/pci/intel-iommu.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

Index: 2.6.21-rc6/drivers/pci/intel-iommu.c
===================================================================
--- 2.6.21-rc6.orig/drivers/pci/intel-iommu.c 2007-04-18 09:04:56.000000000 +0800
+++ 2.6.21-rc6/drivers/pci/intel-iommu.c 2007-04-18 09:04:59.000000000 +0800
@@ -84,7 +84,7 @@ struct iommu {
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 @@ static int __init intel_iommu_setup(char
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 == ',')
@@ -1720,7 +1723,12 @@ static dma_addr_t __intel_map_single(str
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: 2.6.21-rc6/include/linux/intel-iommu.h
===================================================================
--- 2.6.21-rc6.orig/include/linux/intel-iommu.h 2007-04-18 09:04:56.000000000 +0800
+++ 2.6.21-rc6/include/linux/intel-iommu.h 2007-04-18 09:04:59.000000000 +0800
@@ -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)

--