Re: [PATCH] dma-mapping: reliably inform about DMA support for IOMMU

[Christoph Hellwig <hch@lst.de>]

Thanks, I've applied this.  I do like the flow of dma_supported even
less with the duplicate condition now.  Can I get a quick review for
this incremental cleanup, please?

---
From d311bfe189d12a836b4add55fdb946f37f8697fa Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.de>
Date: Thu, 12 Sep 2024 09:21:18 +0200
Subject: dma-mapping: reflow dma_supported

dma_supported has become too much spaghetti for my taste.  Reflow it to
remove the duplicate use_dma_iommu condition and make the main path more
obvious.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 kernel/dma/mapping.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 022d670f8cad29..cd2a97d362cd24 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -841,20 +841,23 @@ 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;
-
-	if (use_dma_iommu(dev))
+	if (use_dma_iommu(dev)) {
+		if (WARN_ON(ops))
+			return false;
 		return true;
+	}
+
 	/*
-	 * ->dma_supported sets the bypass flag, so we must always call
-	 * into the method here unless the device is truly direct mapped.
+	 * ->dma_supported sets and clears the bypass flag, so ignore it here
+	 * and always call into the method if there is one.
 	 */
-	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);
+	}
+
+	return dma_direct_supported(dev, mask);
 }
 
 bool dma_pci_p2pdma_supported(struct device *dev)
-- 
2.45.2