From: Christoph Hellwig <hch@lst.de>
Date: Mon, 1 Oct 2018 14:40:53 +0000 (-0700)
Subject: dma-direct: document the zone selection logic
X-Git-Tag: Ubuntu-5.10.0-12.13~6827^2~2
X-Git-Url: https://git.proxmox.com/?a=commitdiff_plain;h=79ac32a427f5d1211fa417021fd04c36f63ab917;p=mirror_ubuntu-hirsute-kernel.git

dma-direct: document the zone selection logic

What we are doing here isn't quite obvious, so add a comment explaining
it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 674a8da22844..a5a943836c8e 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 physical 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))