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SubjectRe: [PATCH v2 2/4] ARC: allow to use IOC and non-IOC DMA devices simultaneously

On 08/22/2018 11:40 AM, Eugeniy Paltsev wrote:
>> Reading kernel/dma/* I see what you mean. We check @ioc_enable at the time of
>> registering the dma op for coherent vs. non coherent case, so there's common vs.
>> ARC versions of alloc/free for coherent vs. noncoherent.
> Just to be sure that we understand both each other and source code correctly:
> - In coherent case we use dma_direct_* ops which doesn't use ARC alloc functions (arch_dma_{alloc|free})
> - In non-coherent case we use dma_noncoherent_* ops which uses ARC alloc functions (arch_dma_{alloc|free})

Right I see that.

>> But then I'm curious why
>> do we bother to check the following in new arch_dma_(alloc|free) at all.
>> Isn't it supposed to be NON_CONSISTENT always given the way new code works ?
> DMA_ATTR_NON_CONSISTENT flag is not related to IOC topic.
> It is a flag which we can pass to dma_{alloc|free}_attrs function from driver side.
> According to documentation:
> DMA_ATTR_NON_CONSISTENT: Lets the platform to choose to return either
> consistent or non-consistent memory as it sees fit.

Right I'd them mixed up. But then in case of direct dma ops, the attr is simply
ignored in dma_alloc_attrs() -> dma_direct_alloc(). User always gets coherent memory.

> We check this flag in arch_dma_alloc (which are used in non-coherent case) to
> skip MMU mapping if we are advertised that consistency is not required.
> So, actually we can get rid of this flag checking in arch_dma_alloc and
> simply always do MMU mapping to enforce non-cachability and return
> non-cacheable memory even if DMA_ATTR_NON_CONSISTENT is passed.
> But I don't sure we want to do that.
> BTW: dma_alloc_coherent is simply dma_alloc_attrs with attrs == 0.

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