Re: [PATCH v4 2/2] dma: add IOMMU static calls with clear default ops

[Christoph Hellwig <hch@lst.de>]

On Wed, Sep 11, 2024 at 09:43:05AM +0300, Leon Romanovsky wrote:
> Thanks for the report, I'm looking into it. However, it is unclear to me
> why my patch is causing this issue. The change in dma_supported() should
> produce WARN_ON [1] if new path is taken, otherwise, we return to
> previous behavior.

dma-iommu never implemented .dma_supported and thus claims to support
all dma masks.  To restore that behavior we'd need something like the
patch below:

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 7550b5dc5e55df..d23a4d5a6b31a1 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -841,17 +841,19 @@ static int dma_supported(struct device *dev, u64 mask)
 {
 	const struct dma_map_ops *ops = get_dma_ops(dev);
 
-	if (WARN_ON(ops && use_dma_iommu(dev)))
-		return false;
 	/*
 	 * ->dma_supported sets the bypass flag, so we must always call
 	 * into the method here unless the device is truly direct mapped.
 	 */
-	if (!ops)
-		return dma_direct_supported(dev, mask);
-	if (!ops->dma_supported)
-		return 1;
-	return ops->dma_supported(dev, mask);
+	if (ops) {
+		if (!ops->dma_supported)
+			return 1;
+		return ops->dma_supported(dev, mask);
+	}
+
+	if (use_dma_iommu(dev))
+		return true;
+	return dma_direct_supported(dev, mask);
 }
 
 bool dma_pci_p2pdma_supported(struct device *dev)