[PATCH kernel v2 1/2] dma: Allow mixing bypass and normal IOMMU operation

From: Alexey Kardashevskiy <aik@ozlabs.ru>
To: linuxppc-dev@lists.ozlabs.org
Cc: Christoph Hellwig <hch@lst.de>,
	Michael Ellerman <mpe@ellerman.id.au>,
	iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
	Alexey Kardashevskiy <aik@ozlabs.ru>
Subject: [PATCH kernel v2 1/2] dma: Allow mixing bypass and normal IOMMU operation
Date: Tue, 27 Oct 2020 21:18:40 +1100	[thread overview]
Message-ID: <20201027101841.96056-2-aik@ozlabs.ru> (raw)
In-Reply-To: <20201027101841.96056-1-aik@ozlabs.ru>

At the moment we allow bypassing DMA ops only when we can do this for
the entire RAM. However there are configs with mixed type memory
where we could still allow bypassing IOMMU in most cases;
POWERPC with persistent memory is one example.

This adds another check for the bus limit to determine where bypass
can still work and we invoke direct DMA API; when DMA handle is outside
that limit, we fall back to DMA ops.

This adds a CONFIG_DMA_OPS_BYPASS_BUS_LIMIT config option which is off
by default and will be enable for PPC_PSERIES in the following patch.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---
 kernel/dma/mapping.c | 61 ++++++++++++++++++++++++++++++++++++++++++--
 kernel/dma/Kconfig   |  4 +++
 2 files changed, 63 insertions(+), 2 deletions(-)

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 51bb8fa8eb89..0f4f998e6c72 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -137,6 +137,18 @@ static inline bool dma_map_direct(struct device *dev,
 	return dma_go_direct(dev, *dev->dma_mask, ops);
 }
 
+#ifdef CONFIG_DMA_OPS_BYPASS_BUS_LIMIT
+static inline bool can_map_direct(struct device *dev, phys_addr_t addr)
+{
+	return dev->bus_dma_limit >= phys_to_dma(dev, addr);
+}
+
+static inline bool dma_handle_direct(struct device *dev, dma_addr_t dma_handle)
+{
+	return dma_handle >= dev->archdata.dma_offset;
+}
+#endif
+
 dma_addr_t dma_map_page_attrs(struct device *dev, struct page *page,
 		size_t offset, size_t size, enum dma_data_direction dir,
 		unsigned long attrs)
@@ -151,6 +163,11 @@ dma_addr_t dma_map_page_attrs(struct device *dev, struct page *page,
 
 	if (dma_map_direct(dev, ops))
 		addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
+#ifdef CONFIG_DMA_OPS_BYPASS_BUS_LIMIT
+	else if (dev->bus_dma_limit &&
+		 can_map_direct(dev, (phys_addr_t) page_to_phys(page) + offset + size))
+		addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
+#endif
 	else
 		addr = ops->map_page(dev, page, offset, size, dir, attrs);
 	debug_dma_map_page(dev, page, offset, size, dir, addr);
@@ -167,6 +184,10 @@ void dma_unmap_page_attrs(struct device *dev, dma_addr_t addr, size_t size,
 	BUG_ON(!valid_dma_direction(dir));
 	if (dma_map_direct(dev, ops))
 		dma_direct_unmap_page(dev, addr, size, dir, attrs);
+#ifdef CONFIG_DMA_OPS_BYPASS_BUS_LIMIT
+	else if (dev->bus_dma_limit && dma_handle_direct(dev, addr + size))
+		dma_direct_unmap_page(dev, addr, size, dir, attrs);
+#endif
 	else if (ops->unmap_page)
 		ops->unmap_page(dev, addr, size, dir, attrs);
 	debug_dma_unmap_page(dev, addr, size, dir);
@@ -190,6 +211,23 @@ int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg, int nents,
 
 	if (dma_map_direct(dev, ops))
 		ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
+#ifdef CONFIG_DMA_OPS_BYPASS_BUS_LIMIT
+	else if (dev->bus_dma_limit) {
+		struct scatterlist *s;
+		bool direct = true;
+		int i;
+
+		for_each_sg(sg, s, nents, i) {
+			direct = can_map_direct(dev, sg_phys(s) + s->offset + s->length);
+			if (!direct)
+				break;
+		}
+		if (direct)
+			ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
+		else
+			ents = ops->map_sg(dev, sg, nents, dir, attrs);
+	}
+#endif
 	else
 		ents = ops->map_sg(dev, sg, nents, dir, attrs);
 	BUG_ON(ents < 0);
@@ -207,9 +245,28 @@ void dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
 
 	BUG_ON(!valid_dma_direction(dir));
 	debug_dma_unmap_sg(dev, sg, nents, dir);
-	if (dma_map_direct(dev, ops))
+	if (dma_map_direct(dev, ops)) {
 		dma_direct_unmap_sg(dev, sg, nents, dir, attrs);
-	else if (ops->unmap_sg)
+		return;
+	}
+#ifdef CONFIG_DMA_OPS_BYPASS_BUS_LIMIT
+	if (dev->bus_dma_limit) {
+		struct scatterlist *s;
+		bool direct = true;
+		int i;
+
+		for_each_sg(sg, s, nents, i) {
+			direct = dma_handle_direct(dev, s->dma_address + s->length);
+			if (!direct)
+				break;
+		}
+		if (direct) {
+			dma_direct_unmap_sg(dev, sg, nents, dir, attrs);
+			return;
+		}
+	}
+#endif
+	if (ops->unmap_sg)
 		ops->unmap_sg(dev, sg, nents, dir, attrs);
 }
 EXPORT_SYMBOL(dma_unmap_sg_attrs);
diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
index c99de4a21458..02fa174fbdec 100644
--- a/kernel/dma/Kconfig
+++ b/kernel/dma/Kconfig
@@ -20,6 +20,10 @@ config DMA_OPS
 config DMA_OPS_BYPASS
 	bool
 
+# IOMMU driver limited by a DMA window size may switch between bypass and window
+config DMA_OPS_BYPASS_BUS_LIMIT
+	bool
+
 config NEED_SG_DMA_LENGTH
 	bool
 
-- 
2.17.1

