linux-kernel - Re: [PATCH] x86: enable DMA CMA with swiotlb

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Date:	Fri, 22 Nov 2013 21:27:24 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org
Subject: Re: [PATCH] x86: enable DMA CMA with swiotlb

2013/11/22 H. Peter Anvin <hpa@...or.com>:
> On 11/21/2013 07:02 AM, Akinobu Mita wrote:
>>
>> I want to use DMA CMA for the feature that requires a single huge chunk
>> (about 64MB) of host memory.
>
> That is tautological.  "I want to use the Contiguous Memory Allocator to
> allocate contiguous memory."
>
>> Please refer 3 of 24 in:
>> http://www.jedec.org/sites/default/files/T_Fujisawa_MF_2013.pdf
>
> But yes, it makes sense that a device using host memory would not be
> able to leverage swiotlb, because the software simply has no clue what
> the DMA transactions are.  In that sense it is fundamentally different
> from a real IOTLB.

I'm currently working for integrating DMA CMA with intel-iommu which
I can test.  Regardless of which dma ops is actually used, I would
like to allocate large contiguous memory with dma_alloc_coherent()
and tell the base address to the device.

We can use mem= or crashkernel= to reserve host memory, but it is
complicated and unables to work with real IOMMU.
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