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

Tang, Feng
2015-07-31T02:56:17Z

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Hi Michal Nazarewicz,

Thanks for the review.

On Thu, 2015-07-30 at 15:59 +0200, Michal Nazarewicz wrote:
> 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.

Yes! This is exactly what I tried when I first saw this problem, but
this failed as there 2 places inside drivers/base/dma-contiguous.c
which set the dma_contiguous_default_area

1)
void __init dma_contiguous_reserve(phys_addr_t limit)
{
....
dma_contiguous_reserve_area(selected_size, selected_base,
selected_limit,
&dma_contiguous_default_area,
fixed);
....
}

2)
static int __init rmem_cma_setup(struct reserved_mem *rmem)
{
...
if (of_get_flat_dt_prop(node, "linux,cma-default", NULL))
dma_contiguous_set_default(cma);
...
}

Are you suggesting me to remove them?

Thanks,
Feng

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