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Message-ID: <20140310143247.GZ21483@n2100.arm.linux.org.uk>
Date: Mon, 10 Mar 2014 14:32:47 +0000
From: Russell King - ARM Linux <linux@....linux.org.uk>
To: David Woodhouse <dwmw2@...radead.org>
Cc: "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>,
"Koul, Vinod" <vinod.koul@...el.com>,
"Krogerus, Heikki" <heikki.krogerus@...el.com>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Viresh Kumar <viresh.kumar@...com>,
Kukjin Kim <kgene.kim@...sung.com>,
Linus Walleij <linus.walleij@...ricsson.com>,
yuanyabin1978@...a.com, linux-kernel@...r.kernel.org,
Ben Dooks <ben-linux@...ff.org>,
Peter Pearse <peter.pearse@....com>,
Dan Williams <dan.j.williams@...el.com>,
linux-arm-kernel@...ts.infradead.org,
Alessandro Rubini <rubini@...pv.it>
Subject: Re: [PATCH 06/13] DMAENGINE: driver for the ARM PL080/PL081
PrimeCells
On Mon, Mar 10, 2014 at 06:56:30AM -0700, David Woodhouse wrote:
> On Sat, 2011-01-01 at 15:15 +0000, Russell King - ARM Linux wrote:
> > On Wed, Dec 22, 2010 at 03:45:39PM -0800, Dan Williams wrote:
> > > Support for the DMA_COMPL flags are necessary if the DMA_MEMCPY
> > > capability is advertised, yes this driver got this wrong. I'll update
> > > the documentation to make this requirement clear, and audit the other
> > > drivers. With slave-only drivers the only usage model is one where
> > > the client driver owns dma-mapping. In the non-slave (opportunistic
> > > memcpy offload) case the client is unaware of the engine so the driver
> > > owns unmapping. The minimal fix is to disable memcpy offload.
> >
> > As a side note, the DMA mapping for slaves should be done using the
> > DMA struct device, not the struct device of the peripheral making use
> > of the DMA engine.
> >
> > Why? The slave device has no knowledge of how the DMA engine is
> > connected into the system, or the DMA parameters associated with the
> > device performing the DMA, such as the DMA mask and boundaries. (If
> > there are several generic DMA agents in the system, it can't know
> > which is the correct one to use until a channel has been allocated.)
> > The only struct device which has this information is the one for the
> > DMA engine itself.
> >
> > Therefore, the struct device which is passed into the DMA mapping APIs
> > to prepare memory for DMA must always be the DMA engine struct device
> > (chan->device->dev) and never the slave struct device.
>
> That all seems eminently sensible. However, I wonder if it always has to
> be true.
>
> It is not impossible for the DMA controller to "delegate" transactions
> so that (to the IOMMU) they appear to come from the individual slave
> device rather than from itself.
>
> The Intel IOMMU has now gained support for DMA mapping for devices
> enumerated by ACPI — essentially the ACPI "DMAR" table just has a lookup
> table of ACPI device paths, and tells us the PCI bus/devfn that their
> DMA transactions will *appear* to be from.
>
> What we've seen is that it is the individual slave devices that are
> listed in these tables, *not* the DMA controller itself. It looks like
> we are actually expected to set up the IOMMU mapping for the *slave*,
> not the DMA controller. The system doesn't even *tell* me how to set up
> DMA mappings for the DMA controller device; only the slaves.
Okay, so how do you get the DMA address which is to be programmed into
the DMA controller - bearing in mind that different devices in the
system may have different bus:physical offsets?
ACPI may allow you to work this out for each slave device, but now
try thinking about this same problem without ACPI.
--
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