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.
