Re: [PATCH 31/33] dma-direct: reject too small dma masks

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On 10/01/18 08:00, Christoph Hellwig wrote:
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
  include/linux/dma-direct.h |  1 +
  lib/dma-direct.c           | 19 +++++++++++++++++++
  2 files changed, 20 insertions(+)

diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
index 4788bf0bf683..bcdb1a3e4b1f 100644
--- a/include/linux/dma-direct.h
+++ b/include/linux/dma-direct.h
@@ -42,5 +42,6 @@ void *dma_direct_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
  		gfp_t gfp, unsigned long attrs);
  void dma_direct_free(struct device *dev, size_t size, void *cpu_addr,
  		dma_addr_t dma_addr, unsigned long attrs);
+int dma_direct_supported(struct device *dev, u64 mask);
#endif /* _LINUX_DMA_DIRECT_H */
diff --git a/lib/dma-direct.c b/lib/dma-direct.c
index 784a68dfdbe3..40b1f92f2214 100644
--- a/lib/dma-direct.c
+++ b/lib/dma-direct.c
@@ -122,6 +122,24 @@ static int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl,
  	return nents;
  }
+int dma_direct_supported(struct device *dev, u64 mask)
+{
+#ifdef CONFIG_ZONE_DMA
+	if (mask < DMA_BIT_MASK(ARCH_ZONE_DMA_BITS))
+		return 0;
+#else
+	/*
+	 * Because 32-bit DMA masks are so common we expect every architecture
+	 * to be able to satisfy them - either by not supporting more physical
+	 * memory, or by providing a ZONE_DMA32.  If neither is the case, the
+	 * architecture needs to use an IOMMU instead of the direct mapping.
+	 */
+	if (mask < DMA_BIT_MASK(32))
+		return 0;

Do you think it's worth the effort to be a little more accommodating here? i.e.:

		return dma_max_pfn(dev) >= max_pfn;

We seem to have a fair few 28-31 bit masks for older hardware which probably associates with host systems packing equivalently small amounts of RAM.

Otherwise though,

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

Robin.

+#endif
+	return 1;
+}
+
  static int dma_direct_mapping_error(struct device *dev, dma_addr_t dma_addr)
  {
  	return dma_addr == DIRECT_MAPPING_ERROR;
@@ -132,6 +150,7 @@ const struct dma_map_ops dma_direct_ops = {
  	.free			= dma_direct_free,
  	.map_page		= dma_direct_map_page,
  	.map_sg			= dma_direct_map_sg,
+	.dma_supported		= dma_direct_supported,
  	.mapping_error		= dma_direct_mapping_error,
  };
  EXPORT_SYMBOL(dma_direct_ops);




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