linux-kernel - Re: DMA-API attr - DMA_ATTR_NO_KERNEL

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Date:   Mon, 1 Jul 2019 18:54:47 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Pankaj Suryawanshi <pankajssuryawanshi@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: DMA-API attr - DMA_ATTR_NO_KERNEL_MAPPING

On 01/07/2019 18:47, Pankaj Suryawanshi wrote:
>> If you want a kernel mapping, *don't* explicitly request not to have a
>> kernel mapping in the first place. It's that simple.
>>
> 
> Do you mean do not use dma-api ? because if i used dma-api it will give you
> mapped virtual address.
> or i have to use directly cma_alloc() in my driver. // if i used this
> approach i need to reserved more vmalloc area.

No, I mean just call dma_alloc_attrs() normally *without* adding the 
DMA_ATTR_NO_KERNEL_MAPPING flag. That flag means "I never ever want to 
make CPU accesses to this buffer from the kernel" - that is clearly not 
the case for your code, so it is utterly nonsensical to still pass the 
flag but try to hack around it later.

Robin.