linux-kernel - [PATCH] ARM: dma-mapping: don't use the atomic pool for DMA_ATTR_NO_KERNEL

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Date:   Thu,  9 May 2019 09:20:06 +0200
From:   Christoph Hellwig <hch@....de>
To:     Russell King <linux@...linux.org.uk>
Cc:     linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: [PATCH] ARM: dma-mapping: don't use the atomic pool for DMA_ATTR_NO_KERNEL_MAPPING

DMA allocations with the DMA_ATTR_NO_KERNEL_MAPPING do not return a kernel
virtual address for use in driver, but are expected to be used entirely
for userspace mappings and/or device private memory.

Because of that we don't need to remap them as uncached, and thus don't need
the atomic pool for non-blocking allocations.  Note that using the
DMA allocator with DMA_ATTR_NO_KERNEL_MAPPING from non-blocking context
on a non-coherent device is actually broken without this patch as well, as
we feed the address passes to dma_free_attrs directly to the genpool
allocator, but for DMA_ATTR_NO_KERNEL_MAPPING allocations it actually
contains the address of the first page pointer.

Signed-off-by: Christoph Hellwig <hch@....de>
---
 arch/arm/mm/dma-mapping.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index d029fa31624b..f74d824f8fc1 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -706,6 +706,7 @@ static void *__dma_alloc(struct device *dev, size_t size, dma_addr_t *handle,
 			 gfp_t gfp, pgprot_t prot, bool is_coherent,
 			 unsigned long attrs, const void *caller)
 {
+	bool want_vaddr = !(attrs & DMA_ATTR_NO_KERNEL_MAPPING);
 	u64 mask = get_coherent_dma_mask(dev);
 	struct page *page = NULL;
 	void *addr;
@@ -717,7 +718,7 @@ static void *__dma_alloc(struct device *dev, size_t size, dma_addr_t *handle,
 		.gfp = gfp,
 		.prot = prot,
 		.caller = caller,
-		.want_vaddr = ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) == 0),
+		.want_vaddr = want_vaddr,
 		.coherent_flag = is_coherent ? COHERENT : NORMAL,
 	};
 
@@ -755,14 +756,14 @@ static void *__dma_alloc(struct device *dev, size_t size, dma_addr_t *handle,
 	allowblock = gfpflags_allow_blocking(gfp);
 	cma = allowblock ? dev_get_cma_area(dev) : false;
 
-	if (cma)
+	if (!allowblock && !is_coherent && want_vaddr)
+		buf->allocator = &pool_allocator;
+	else if (cma)
 		buf->allocator = &cma_allocator;
 	else if (is_coherent)
 		buf->allocator = &simple_allocator;
-	else if (allowblock)
-		buf->allocator = &remap_allocator;
 	else
-		buf->allocator = &pool_allocator;
+		buf->allocator = &remap_allocator;
 
 	addr = buf->allocator->alloc(&args, &page);
 
-- 
2.20.1