Re: [Linaro-mm-sig] [PATCH 4/8] dma-buf: add peer2peer flag

[Christoph Hellwig <hch@infradead.org>]

On Wed, Apr 25, 2018 at 09:02:17AM +0200, Daniel Vetter wrote:
> Can we please not nack everything right away? Doesn't really motivate
> me to show you all the various things we're doing in gpu to make the
> dma layer work for us. That kind of noodling around in lower levels to
> get them to do what we want is absolutely par-for-course for gpu
> drivers. If you just nack everything I point you at for illustrative
> purposes, then I can't show you stuff anymore.

No, it's not.  No driver (and that includes the magic GPUs) has
any business messing with dma ops directly.

A GPU driver imght have a very valid reason to disable the IOMMU,
but the code to do so needs to be at least in the arch code, maybe
in the dma-mapping/iommu code, not in the driver.

As a first step to get the discussion started we'll simply need
to move the code Thierry wrote into a helper in arch/arm and that
alone would be a massive improvement.  I'm not even talking about
minor details like actually using arm_get_dma_map_ops instead
of duplicating it.

And doing this basic trivial work really helps to get this whole
mess under control.