Re: [PATCH 2/3] dma: Introduce dma_max_mapping_size()

[Christoph Hellwig <hch@lst.de>]

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.