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

From: Christoph Hellwig
Date: Wed Feb 06 2019 - 02:07:32 EST

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;

- 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);