Re: [PATCH v2 0/7] of: setup dma parameters using dma-ranges and dma-coherent

[Catalin Marinas]

On Tue, Apr 22, 2014 at 04:02:19PM +0100, Arnd Bergmann wrote:
> On Saturday 19 April 2014, Thomas Petazzoni wrote:
> > I am not sure whether there is an intersection or not, but I wanted to
> > mention that the mvebu platform (in mach-mvebu) supports hardware I/O
> > coherency, which makes it a coherent DMA platform. However, we are not
> > able to use arm_coherent_dma_ops for this platform, because when a
> > transfer is being made DMA_FROM_DEVICE, at the end of the transfer, we
> > need to perform an I/O barrier to wait for the snooping unit to
> > complete its coherency work. So we're coherent, but not with
> > arm_coherent_dma_ops: we have our own dma operation implementation (see
> > arch/arm/mach-mvebu/coherency.c).
> 
> I had completely missed the fact that this support was merged already.
> 
> It's an interesting question if this should actually be called
> 'coherent' or not. It's certainly more coherent than without that
> support, but then again, you still can't rely on incoming data to
> be visible after a readl() from the device has returned or an MSI
> interrupt has been delivered, which is what we normally expect.
> 
> In particular, it means you can't really use arm_coherent_dma_alloc(),
> which is a shame, since that is a significante performance overhead.

It should still work if __io*mb() macros do the extra work specific to
mvebu (similar to the L2x0 outer_sync(), though this was for
non-cacheable DMA buffers).

> I would hope we can find a way to avoid the platform notifiers for
> mvebu as well and come up with a generic way to express this
> 'semi-coherent' mode. I believe x-gene has a similar issue, and
> I wouldn't be surprised if there are others like this.

The solution is for the snooping unit to detect the DSB instruction
(which is propagated outside the CPU) and wait for the completion of the
coherency work (but we need more information from the hardware guys).

-- 
Catalin
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