linux-kernel - Re: [PATCH v2] dma-direct: do not allocate a single page from CMA area

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Date:   Wed, 6 Feb 2019 08:07:26 +0100
From:   Christoph Hellwig <hch@....de>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     Christoph Hellwig <hch@....de>, m.szyprowski@...sung.com,
        robin.murphy@....com, vdumpa@...dia.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] dma-direct: do not allocate a single page from CMA
 area

On Tue, Feb 05, 2019 at 03:05:30PM -0800, Nicolin Chen wrote:
> > And my other concern is that this skips allocating from the per-device
> > pool, which drivers might rely on.
> 
> Actually Robin had the same concern at v1 and suggested that we could
> always use DMA_ATTR_FORCE_CONTIGUOUS to enforce into per-device pool.

That is both against the documented behavior of DMA_ATTR_FORCE_CONTIGUOUS
and doesn't help existing drivers that specify their CMA area in DT.

> > To be honest I'm not sure there is
> > much of a point in the per-device CMA pool vs the traditional per-device
> > coherent pool, but I'd rather change that behavior in a clearly documented
> > commit with intentions rather as a side effect from a random optimization.
> 
> Hmm..sorry, I don't really follow this suggestion. Is it possible for
> you to make it clear that what should I do for the change?

Something like this (plus proper comments):

diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
index b2a87905846d..789d734f0f77 100644
--- a/kernel/dma/contiguous.c
+++ b/kernel/dma/contiguous.c
@@ -192,10 +192,19 @@ int __init dma_contiguous_reserve_area(phys_addr_t size, phys_addr_t base,
 struct page *dma_alloc_from_contiguous(struct device *dev, size_t count,
 				       unsigned int align, bool no_warn)
 {
+	struct cma *cma;
+
 	if (align > CONFIG_CMA_ALIGNMENT)
 		align = CONFIG_CMA_ALIGNMENT;
 
-	return cma_alloc(dev_get_cma_area(dev), count, align, no_warn);
+	if (dev && dev->cma_area)
+		cma = dev->cma_area;
+	else if (count > PAGE_SIZE)
+		cma = dma_contiguous_default_area;
+	else
+		return NULL;
+
+	return cma_alloc(cma, count, align, no_warn);
 }
 
 /**