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

[Michal Nazarewicz <mina86@mina86.com>]

On Fri, Jul 31 2015, Feng Tang wrote:
> 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?

Set CONFIG_CMA_SIZE_MBYTES to zero (in Kconfig) or add ‘cma=0’ on
command line, and don’t use ‘linux,cma-default’ in device tree.  Then
you will end up with no default CMA area.

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