[PATCH] ARM: mm: dma: Update coherent streaming apis with missing memory barrier

[Arnd Bergmann]

On Wednesday 23 April 2014 19:37:42 Russell King - ARM Linux wrote:
> On Wed, Apr 23, 2014 at 06:17:27PM +0100, Will Deacon wrote:
> > On Wed, Apr 23, 2014 at 05:02:16PM +0100, Catalin Marinas wrote:
> > > In the I/O coherency case, I would say it is the responsibility of the
> > > device/hardware to ensure that the data is visible to all observers
> > > (CPUs) prior to issuing a interrupt for DMA-ready. Looking at the mvebu
> > > code, I think it covers such scenario from-device or bidirectional
> > > scenarios.
> > > 
> > > Maybe Santosh still has a point  but I don't know what the right
> > > barrier would be here. And I really *hate* per-SoC/snoop unit barriers
> > > (I still hope a dsb would do the trick on newer/ARMv8 systems).
> > 
> > If you have device interrupts which are asynchronous to memory coherency,
> > then you're in a world of pain. I can't think of a generic (architected)
> > solution to this problem, unfortunately -- it's going to be both device
> > and interconnect specific. Adding dsbs doesn't necessarily help at all.
> 
> Think, network devices with NAPI handling.  There, we explicitly turn
> off the device's interrupt, and switch to software polling for received
> packets.
>
> The memory for the packets has already been mapped, and we're unmapping
> the buffer, and then reading from it (to locate the ether type, and/or
> vlan headers) before passing it up the network stack.
> 
> So in this case, we need to ensure that the cache operations are ordered
> before the subsequent loads read from the DMA'd data.  It's purely an
> ordering thing, it's not a completion thing.

PCI guarantees this, but I have seen systems in the past (on PowerPC) that
would violate them on the internal interconnect: You could sometimes see the
completion DMA data in the descriptor ring before the actual user data
is there. We only ever observed it in combination with an IOMMU, when the
descriptor address had a valid IOTLB but the data address did not.

I would hope that the ARM SMMU gets this right, but there are also
a number of other IOMMU implementations.

The x-gene SATA driver apparently suffers from a related problem, and
they have to flush outstanding DMAs at the interconnect whenever they
get a completion interrupt.

Another problem is MSI processing. MSI was specifically invented to avoid
having to check an MMIO register for a DMA completion that as a side-effect
flushes pending DMAs from the same device. This breaks down if the MSI
packet gets turned into a level interrupt before it reaches the CPU's
coherency domain, which is likely the case on the dw-pcie controller that
comes with its own MSI block.

	Arnd