linux-kernel - [PATCH] dma-contiguous: Prioritize restricted DMA pool over shared DMA pool

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Date:   Tue, 15 Feb 2022 14:43:44 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     bcm-kernel-feedback-list@...adcom.com, jim2101024@...il.com,
        opendmb@...il.com, robh@...nel.org, will@...nel.org,
        tientzu@...omium.org, Florian Fainelli <f.fainelli@...il.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        iommu@...ts.linux-foundation.org (open list:DMA MAPPING HELPERS)
Subject: [PATCH] dma-contiguous: Prioritize restricted DMA pool over shared DMA pool

Some platforms might define the same memory region to be suitable for
used by a kernel supporting CONFIG_DMA_RESTRICTED_POOL while maintaining
compatibility with older kernels that do not support that. This is
achieved by declaring the node this way;

	cma@...00000 {
		compatible = "restricted-dma-pool", "shared-dma-pool";
		reusable;
		...
	};

A newer kernel would leverage the 'restricted-dma-pool' compatible
string for that reserved region, while an older kernel would use the
'shared-dma-pool' compatibility string.

Due to the link ordering between files under kernel/dma/ however,
contiguous.c would try to claim the region and we would never get a
chance for swiotlb.c to claim that reserved memory region.

To that extent, update kernel/dma/contiguous.c in order to check
specifically for the 'restricted-dma-pool' compatibility string when
CONFIG_DMA_RESTRICTED_POOL is enabled and give up that region such that
kernel/dma/swiotlb.c has a chance to claim it.

Similarly, kernel/dma/swiotlb.c is updated to remove the check for the
'reusable' property because that check is not useful. When a region is
defined with a compatible string set to 'restricted-dma-pool', no code
under kernel/dma/{contiguous,coherent}.c will match that region since
there is no 'shared-dma-pool' compatible string. If a
region is defined with a compatible string set as above with both
'restricted-dma-pool" *and* 'shared-dma-pool' however, checking for
'reusable' will prevent kernel/dma/swiotlb.c from claiming the region
while it is still perfectly suitable since contiguous.c gave it up.

Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
---
 kernel/dma/contiguous.c | 7 +++++++
 kernel/dma/swiotlb.c    | 3 +--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
index 3d63d91cba5c..3c418af6d306 100644
--- a/kernel/dma/contiguous.c
+++ b/kernel/dma/contiguous.c
@@ -416,6 +416,13 @@ static int __init rmem_cma_setup(struct reserved_mem *rmem)
 	    of_get_flat_dt_prop(node, "no-map", NULL))
 		return -EINVAL;

+#ifdef CONFIG_DMA_RESTRICTED_POOL
+	if (of_flat_dt_is_compatible(node, "restricted-dma-pool")) {
+		pr_warn("Giving up %pa for restricted DMA pool\n", &rmem->base);
+		return -ENOENT;
+	}
+#endif
+
 	if ((rmem->base & mask) || (rmem->size & mask)) {
 		pr_err("Reserved memory: incorrect alignment of CMA region\n");
 		return -EINVAL;
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index f1e7ea160b43..9d6e4ae74d04 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -883,8 +883,7 @@ static int __init rmem_swiotlb_setup(struct reserved_mem *rmem)
 {
 	unsigned long node = rmem->fdt_node;

-	if (of_get_flat_dt_prop(node, "reusable", NULL) ||
-	    of_get_flat_dt_prop(node, "linux,cma-default", NULL) ||
+	if (of_get_flat_dt_prop(node, "linux,cma-default", NULL) ||
 	    of_get_flat_dt_prop(node, "linux,dma-default", NULL) ||
 	    of_get_flat_dt_prop(node, "no-map", NULL))
 		return -EINVAL;
-- 
2.25.1