linux-kernel - Re: [RFC PATCH] dma/swiotlb: Add helper for device driver to opt-out from swiotlb.

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Date:	Thu, 17 Sep 2015 15:07:47 -0400
From:	Jerome Glisse <jglisse@...hat.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	iommu@...ts.linux-foundation.org, Joerg Roedel <jroedel@...e.de>,
	Alex Deucher <alexander.deucher@....com>,
	Dave Airlie <airlied@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] dma/swiotlb: Add helper for device driver to opt-out
 from swiotlb.

On Thu, Sep 17, 2015 at 03:02:51PM -0400, Konrad Rzeszutek Wilk wrote:
> On Thu, Sep 17, 2015 at 02:22:38PM -0400, jglisse@...hat.com wrote:
> > From: Jérôme Glisse <jglisse@...hat.com>
> > 
> > The swiotlb dma backend is not appropriate for some devices like
> > GPU where bounce buffer or slow dma page allocations is just not
> > acceptable. With that helper device drivers can opt-out from the
> > swiotlb and just do sane things without wasting CPU cycles inside
> > the swiotlb code.
> 
> What if SWIOTLB is the only one available?

On x86 no_mmu is always available and we assume that device driver
that would use this knows that their device can access all memory
with no restriction or at very least use DMA32 gfp flag.

> And what can't the devices use the TTM DMA backend which sets up
> buffers which don't need bounce buffer or slow dma page allocations?

We want to get rid of this TTM code path for radeon and likely
nouveau. This is the motivation for that patch. Benchmark shows
that the TTM DMA backend is much much much slower (20% on some
benchmark) that the regular page allocation and going through
no_mmu.

So this is all about allowing to directly allocate page through
regular kernel page alloc code and not through specialize dma
allocator.

Cheers,
Jérôme
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