Re: [PATCH] CMA: Don't return a valid cma for non-cma dev

Michal Nazarewicz
2015-07-30T13:59:51Z

On Thu, Jul 30 2015, Feng Tang wrote:
> When system(one x86 soc) boot, we saw many normal dma allocation requests
> goes to cma area. The call chain is
> dma_generic_alloc_coherent
> dma_alloc_from_contiguous -- arch/x86/kernel/pci-dma.c
> cma_alloc(dev_get_cma_area(dev), count, align)
>
> Current dev_get_cma_area() will return a valid "cma" anyway. Then all
> these requests will be taken as valid cma request, and get pages from
> cma area, which has 2 problems:
> 1. make the cma area fragmented
> 2. confuse the cma reservation, usually cma memory size is set according
> to the expectation of system scenario, these unexpected requests
> will affect the designed cma usage.
>
> So this patch will enforce the judgement, and only return valid "cma"
> for real cma user, thus make normal user like IO device driver not
> abuse cma reserved region.

Just don’t set dma_contiguous_default_area. This patch defeats the
purpose of a *default* area.

> Signed-off-by: Feng Tang <feng.tang@intel.com>
> ---
> include/linux/dma-contiguous.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/dma-contiguous.h b/include/linux/dma-contiguous.h
> index 569bbd0..d6ccc19 100644
> --- a/include/linux/dma-contiguous.h
> +++ b/include/linux/dma-contiguous.h
> @@ -66,7 +66,8 @@ static inline struct cma *dev_get_cma_area(struct device *dev)
> {
> if (dev && dev->cma_area)
> return dev->cma_area;
> - return dma_contiguous_default_area;
> + else
> + return NULL;
> }
>
> static inline void dev_set_cma_area(struct device *dev, struct cma *cma)
> --
> 1.7.9.5
>

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