linux-kernel - [RFC] arm64: swiotlb: cma

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Date:   Wed, 17 Apr 2019 14:48:17 -0600
From:   dann frazier <dann.frazier@...onical.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Xinwei Kong <kong.kongxinwei@...ilicon.com>
Subject: [RFC] arm64: swiotlb: cma_alloc error spew

hey,
  I'm seeing an issue on a couple of arm64 systems[*] where they spew
~10K "cma: cma_alloc: alloc failed" messages at boot. The errors are
non-fatal, and bumping up cma to a large enough size (~128M) gets rid
of them - but that seems suboptimal. Bisection shows that this started
after commit fafadcd16595 ("swiotlb: don't dip into swiotlb pool for
coherent allocations"). It looks like __dma_direct_alloc_pages()
is opportunistically using CMA memory but falls back to non-CMA if CMA
disabled or unavailable. I've demonstrated that this fallback is
indeed returning a valid pointer. So perhaps the issue is really just
the warning emission.

The following naive patch solves the problem for me - just silence the
cma errors, since it looks like a soft error. But is there a better
approach?

[*] APM X-Gene & HiSilicon Hi1620 w/ SMMU disabled

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 6310ad01f915b..0324aa606c173 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -112,7 +112,7 @@ struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
        /* CMA can be used only in the context which permits sleeping */
        if (gfpflags_allow_blocking(gfp)) {
                page = dma_alloc_from_contiguous(dev, count, page_order,
-                                                gfp & __GFP_NOWARN);
+                                                true);
                if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
                        dma_release_from_contiguous(dev, page, count);
                        page = NULL;