linux-kernel - Re: [PATCH 6/9] crypto: ux500/cryp - Set DMA configuration though dma_slave

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Date:	Fri, 26 Apr 2013 10:41:23 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Lee Jones <lee.jones@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus WALLEIJ <linus.walleij@...ricsson.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"David S. Miller" <davem@...emloft.net>,
	Andreas Westin <andreas.westin@...ricsson.com>,
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
	magnus.p.persson@...ricsson.com
Subject: Re: [PATCH 6/9] crypto: ux500/cryp - Set DMA configuration though
	dma_slave_config()

On Fri, Apr 26, 2013 at 01:46:46PM +0530, Vinod Koul wrote:
> On Fri, Apr 26, 2013 at 10:28:39AM +0200, Linus Walleij wrote:
> > On Thu, Apr 25, 2013 at 4:11 PM, Arnd Bergmann <arnd@...db.de> wrote:
> > 
> > > The dma engine driver must know the address in its dma space, while the
> > > slave driver has it available in physical space. These two are often the
> > > same, but there is no generic way to convert between the two, especially
> > > if the dma engine resides behind an IOMMU.
> > >
> > > The best assumption we can make is that the dma engine driver knows
> > > how to convert between the two. Interestingly the documentation for
> > > dma_slave_config talks about "physical address", while the structure
> > > itself uses a dma_addr_t. Linus Walleij introduced the structure in
> > > c156d0a5b0 "DMAENGINE: generic slave channel control v3", so I assume
> > > he can shed some light on what he was thinking. I assume the documentation
> > > is right but the structure is not and should be converted to use
> > > phys_add_t or resource_size_t.
> > 
> > OK I could cook a patch for that, but I think I need some input from
> > Vinod and/or Russell on this.
> the dma_slave_config is physical address that should be passed directly to the
> controller. Obviosuly it should phys_addr_t :)

What you've just said is actually confusing.

"physical address" is normally the term used to describe the addresses
seen to the RAM.  phys_addr_t describes this.  This is not necessarily
what needs to be programmed into the DMA controller.

For RAM addresses, they must be mapped via the DMA API - and this gives
you a dma_addr_t.

"DMA address" is the address to be programmed into a DMA controller to
access a particular address in RAM or device, and has type dma_addr_t.
When you're programming a DMA controller to access a device, you are
clearly telling it the address on the _DMA controller's bus_ to access
that register, which may or may not be the same as the physical address.

There are platforms in existence where phys_addr_t can be 32-bit but
dma_addr_t can be 64-bit.  Getting this stuff wrong can cause problems.
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