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

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Message-ID: <C2D7FE5348E1B147BCA15975FBA23075012B09164B@us01wembx1.internal.synopsys.com>
Date:   Tue, 4 Sep 2018 18:13:28 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
CC:     "hch@....de" <hch@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "Alexey.Brodkin@...opsys.com" <Alexey.Brodkin@...opsys.com>
Subject: Re: [PATCH v2 2/4] ARC: allow to use IOC and non-IOC DMA devices
 simultaneously

Hi,

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.
>>
>> 	if (attrs & DMA_ATTR_NON_CONSISTENT)
>>
>> 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.
>