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

[Arnd Bergmann]

On Thursday 24 April 2014 11:58:46 Will Deacon wrote:
> 
> On Wed, Apr 23, 2014 at 07:58:05PM +0100, Arnd Bergmann wrote:
> > 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.
> 
> I'm not sure there's anything special about MSI which helps with this
> problem. For GICv3, the MSI write will target the ITS (a slave device),
> whereas the data produced is assumedly targetting main memory. That still
> requires careful ordering by the producer, in the same way as if it was
> signalling a legacy interrupt.

With legacy interrupts a PCI bus master has no way to order the transactions:
It initiates the DMA to memory, but does not wait for the DMA to complete.
It raises the interrupt line, which causes the interrupt handler of the
driver to start. The driver then reads a status register from the bus master
device, and this read is ordered with respect to the DMA that may still
be in progress at the time. Any PCI driver that works with legacy interrupts
and DMA has to do this, and the PCI host controller has to ensure that these
ordering semantics are maintained on the upstream buses.

The difference with MSI is that the driver does not have to do an MMIO read
transaction, and that the host controller has to ensure ordering between
the (possibly weakly ordered) data DMA and the MSI transaction, rather than
between the DMA and the MMIO read. These two are not the same, and it's
totally possible for a broken implementation to get one of them right
but not the other.

	Arnd