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

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Message-ID: <20150917192424.GB21496@x230.dumpdata.com>
Date:	Thu, 17 Sep 2015 15:24:25 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Jerome Glisse <jglisse@...hat.com>
Cc:	Alex Deucher <alexander.deucher@....com>,
	Dave Airlie <airlied@...hat.com>,
	iommu@...ts.linux-foundation.org, Joerg Roedel <jroedel@...e.de>,
	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 03:11:14PM -0400, Jerome Glisse wrote:
> On Thu, Sep 17, 2015 at 03:06:57PM -0400, Konrad Rzeszutek Wilk wrote:
> > 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?
> > > 
> > > 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?
> > 
> > And then the followup question. If it opts out - how can it do
> > sane things without an DMA API available? It would assume physical
> > addresses match the bus addresses which is not always the sane
> > thing.
> 
> This is why this is an arch specific function, on x86 with pci device,
> the driver knows what is the dma mask and thus if it can access directly
> all the memory or not. So in the end swiotlb vs no_mmu gives the same
> physical address to the device so there is no difference there.

Not with Intel or AMD IOMMUs. The bus address it gives is not the same
as the physical address.
> 
> Obviously device driver needs to know what it is doing depending on the
> arch and bus the device is use in.
> 
> Cheers,
> Jérôme
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