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

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Message-ID: <20240911080445.GA5950@lst.de>
Date: Wed, 11 Sep 2024 10:04:45 +0200
From: Christoph Hellwig <hch@....de>
To: Leon Romanovsky <leon@...nel.org>
Cc: Nícolas F. R. A. Prado <nfraprado@...labora.com>,
	Christoph Hellwig <hch@....de>, Robin Murphy <robin.murphy@....com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Easwar Hariharan <eahariha@...ux.microsoft.com>,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	Jason Gunthorpe <jgg@...dia.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	regressions@...ts.linux.dev, kernelci@...ts.linux.dev,
	kernel@...labora.com
Subject: Re: [PATCH v4 2/2] dma: add IOMMU static calls with clear default
 ops

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)