On Tue, Jan 15, 2019 at 05:23:22PM +0100, Joerg Roedel wrote:

Right, I thought about that too, but didn't find a generic way to check
for all the cases. There are various checks that could be done:

	1) Check if SWIOTLB is initialized at all, if not, return
	   SIZE_MAX as the limit. This can't be checked from dma-direct
	   code right now, but could be easily implemented.

Yes, this is the low hanging fruit.

	2) Check for swiotlb=force needs to be done.

	3) Check whether the device can access all of available RAM. I
	   have no idea how to check that in an architecture independent
	   way. It also has to take memory hotplug into account as well
	   as the DMA mask of the device.

	   An easy approximation could be to omit the limit if the
	   dma-mask covers all of the physical address bits available
	   on the platform. It would require to pass the dma-mask as an
	   additional parameter like it is done in dma_supported().

Any better ideas for how to implement 3)?

And yeah, this is hard.  So I'd just go for the low hanging fruit
for now and only implement 1) with a comment mentioning that
we are a little pessimistic.