Page mapping could overwritten by an accident (a bug). We can catch this
case by checking 'VALID' bit of GART's page entry prior to mapping of a
page. Since that check introduces a small performance impact, it should be
enabled explicitly using new GART's kernel module 'debug' parameter.

Signed-off-by: Dmitry Osipenko <dig...@gmail.com>
---
 drivers/iommu/tegra-gart.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/iommu/tegra-gart.c b/drivers/iommu/tegra-gart.c
index b62f790ad1ba..4c0abdcd1ad2 100644
--- a/drivers/iommu/tegra-gart.c
+++ b/drivers/iommu/tegra-gart.c
@@ -72,6 +72,8 @@ struct gart_domain {
 
 static struct gart_device *gart_handle; /* unique for a system */
 
+static bool gart_debug;
+
 #define GART_PTE(_pfn)                                         \
        (GART_ENTRY_PHYS_ADDR_VALID | ((_pfn) << PAGE_SHIFT))
 
@@ -271,6 +273,7 @@ static int gart_iommu_map(struct iommu_domain *domain, 
unsigned long iova,
        struct gart_device *gart = gart_domain->gart;
        unsigned long flags;
        unsigned long pfn;
+       unsigned long pte;
 
        if (!gart_iova_range_valid(gart, iova, bytes))
                return -EINVAL;
@@ -282,6 +285,14 @@ static int gart_iommu_map(struct iommu_domain *domain, 
unsigned long iova,
                spin_unlock_irqrestore(&gart->pte_lock, flags);
                return -EINVAL;
        }
+       if (gart_debug) {
+               pte = gart_read_pte(gart, iova);
+               if (pte & GART_ENTRY_PHYS_ADDR_VALID) {
+                       spin_unlock_irqrestore(&gart->pte_lock, flags);
+                       dev_err(gart->dev, "Page entry is in-use\n");
+                       return -EBUSY;
+               }
+       }
        gart_set_pte(gart, iova, GART_PTE(pfn));
        FLUSH_GART_REGS(gart);
        spin_unlock_irqrestore(&gart->pte_lock, flags);
@@ -515,7 +526,9 @@ static void __exit tegra_gart_exit(void)
 
 subsys_initcall(tegra_gart_init);
 module_exit(tegra_gart_exit);
+module_param(gart_debug, bool, 0644);
 
+MODULE_PARM_DESC(gart_debug, "Enable GART debugging");
 MODULE_DESCRIPTION("IOMMU API for GART in Tegra20");
 MODULE_AUTHOR("Hiroshi DOYU <hd...@nvidia.com>");
 MODULE_ALIAS("platform:tegra-gart");
-- 
2.16.3

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