powerpc/iommu: bypass DMA APIs for coherent allocations for pre-mapped memory

Leverage ARCH_HAS_DMA_MAP_DIRECT config option for coherent allocations as
well. This will bypass DMA ops for memory allocations that have been
pre-mapped.

Always set device bus_dma_limit when memory is pre-mapped. In some
architectures, like PowerPC, pmemory can be converted to regular memory via
daxctl command. This will gate the coherent allocations to pre-mapped RAM
only, by dma_coherent_ok().

Signed-off-by: Gaurav Batra <gbatra@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20251107161105.85999-1-gbatra@linux.ibm.com

Author: Gaurav Batra

arch/powerpc/kernel/dma-iommu.c

@@ -65,6 +65,21 @@ bool arch_dma_unmap_sg_direct(struct device *dev, struct scatterlist *sg,
 
 	return true;
 }
+bool arch_dma_alloc_direct(struct device *dev)
+{
+	if (dev->dma_ops_bypass)
+		return true;
+
+	return false;
+}
+
+bool arch_dma_free_direct(struct device *dev, dma_addr_t dma_handle)
+{
+	if (!dev->dma_ops_bypass)
+		return false;
+
+	return is_direct_handle(dev, dma_handle);
+}
 #endif /* CONFIG_ARCH_HAS_DMA_MAP_DIRECT */
 
 /*
@@ -146,17 +161,12 @@ int dma_iommu_dma_supported(struct device *dev, u64 mask)
 
 	if (dev_is_pci(dev) && dma_iommu_bypass_supported(dev, mask)) {
 		/*
-		 * dma_iommu_bypass_supported() sets dma_max when there is
-		 * 1:1 mapping but it is somehow limited.
-		 * ibm,pmemory is one example.
+		 * fixed ops will be used for RAM. This is limited by
+		 * bus_dma_limit which is set when RAM is pre-mapped.
 		 */
-		dev->dma_ops_bypass = dev->bus_dma_limit == 0;
-		if (!dev->dma_ops_bypass)
-			dev_warn(dev,
-				 "iommu: 64-bit OK but direct DMA is limited by %llx\n",
-				 dev->bus_dma_limit);
-		else
-			dev_dbg(dev, "iommu: 64-bit OK, using fixed ops\n");
+		dev->dma_ops_bypass = true;
+		dev_info(dev, "iommu: 64-bit OK but direct DMA is limited by %llx\n",
+			 dev->bus_dma_limit);
 		return 1;
 	}
 

arch/powerpc/platforms/pseries/iommu.c

@@ -1769,10 +1769,8 @@ static bool enable_ddw(struct pci_dev *dev, struct device_node *pdn, u64 dma_mas
 out_unlock:
 	mutex_unlock(&dma_win_init_mutex);
 
-	/* If we have persistent memory and the window size is not big enough
-	 * to directly map both RAM and vPMEM, then we need to set DMA limit.
-	 */
-	if (pmem_present && direct_mapping && len != MAX_PHYSMEM_BITS)
+	/* For pre-mapped memory, set bus_dma_limit to the max RAM */
+	if (direct_mapping)
 		dev->dev.bus_dma_limit = dev->dev.archdata.dma_offset +
 						(1ULL << max_ram_len);
 

include/linux/dma-map-ops.h

@@ -395,11 +395,15 @@ bool arch_dma_map_sg_direct(struct device *dev, struct scatterlist *sg,
 		int nents);
 bool arch_dma_unmap_sg_direct(struct device *dev, struct scatterlist *sg,
 		int nents);
+bool arch_dma_alloc_direct(struct device *dev);
+bool arch_dma_free_direct(struct device *dev, dma_addr_t dma_handle);
 #else
 #define arch_dma_map_phys_direct(d, a)		(false)
 #define arch_dma_unmap_phys_direct(d, a)	(false)
 #define arch_dma_map_sg_direct(d, s, n)		(false)
 #define arch_dma_unmap_sg_direct(d, s, n)	(false)
+#define arch_dma_alloc_direct(d)            (false)
+#define arch_dma_free_direct(d, a)          (false)
 #endif
 
 #ifdef CONFIG_ARCH_HAS_SETUP_DMA_OPS

kernel/dma/mapping.c

@@ -638,7 +638,7 @@ void *dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
 	/* let the implementation decide on the zone to allocate from: */
 	flag &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);
 
-	if (dma_alloc_direct(dev, ops)) {
+	if (dma_alloc_direct(dev, ops) || arch_dma_alloc_direct(dev)) {
 		cpu_addr = dma_direct_alloc(dev, size, dma_handle, flag, attrs);
 	} else if (use_dma_iommu(dev)) {
 		cpu_addr = iommu_dma_alloc(dev, size, dma_handle, flag, attrs);
@@ -679,7 +679,7 @@ void dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
 		return;
 
 	debug_dma_free_coherent(dev, size, cpu_addr, dma_handle);
-	if (dma_alloc_direct(dev, ops))
+	if (dma_alloc_direct(dev, ops) || arch_dma_free_direct(dev, dma_handle))
 		dma_direct_free(dev, size, cpu_addr, dma_handle, attrs);
 	else if (use_dma_iommu(dev))
 		iommu_dma_free(dev, size, cpu_addr, dma_handle, attrs);