From: Christoph Hellwig <hch@lst.de>
To: iommu@lists.linux-foundation.org
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
Robin Murphy <robin.murphy@arm.com>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
linux-kernel@vger.kernel.org
Subject: [PATCH] dma-direct: document the zone selection logic
Date: Mon, 1 Oct 2018 13:10:16 -0700 [thread overview]
Message-ID: <20181001201016.32639-2-hch@lst.de> (raw)
In-Reply-To: <20181001201016.32639-1-hch@lst.de>
What we are doing here isn't quite obvious, so add a comment explaining
it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
kernel/dma/direct.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index ba6f5956a291..14b966e2349a 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -84,7 +84,14 @@ static gfp_t __dma_direct_optimal_gfp_mask(struct device *dev, u64 dma_mask,
else
*phys_mask = dma_to_phys(dev, dma_mask);
- /* GFP_DMA32 and GFP_DMA are no ops without the corresponding zones: */
+ /*
+ * Optimistically try the zone that the physicall address mask falls
+ * into first. If that returns memory that isn't actually addressable
+ * we will fallback to the next lower zone and try again.
+ *
+ * Note that GFP_DMA32 and GFP_DMA are no ops without the corresponding
+ * zones.
+ */
if (*phys_mask <= DMA_BIT_MASK(ARCH_ZONE_DMA_BITS))
return GFP_DMA;
if (*phys_mask <= DMA_BIT_MASK(32))
--
2.19.0
next prev parent reply other threads:[~2018-10-01 20:11 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-01 20:10 document dma-direct zone selection Christoph Hellwig
2018-10-01 20:10 ` Christoph Hellwig [this message]
2018-10-01 20:22 ` [PATCH] dma-direct: document the zone selection logic Randy Dunlap
2018-10-08 7:03 ` Christoph Hellwig
2018-10-08 22:56 ` Benjamin Herrenschmidt
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