On Mon, Mar 23, 2020 at 12:28:34PM +1100, Alexey Kardashevskiy wrote:
[full quote deleted, please follow proper quoting rules]
> > +static bool dma_alloc_direct(struct device *dev, const struct dma_map_ops
> > *ops)
> > +{
> > + if (!ops)
> > + return true;
> > +
> > + /*
> > + * Allows IOMMU drivers to bypass dynamic translations if the DMA mask
> > + * is large enough.
> > + */
> > + if (dev->dma_ops_bypass) {
> > + if (min_not_zero(dev->coherent_dma_mask, dev->bus_dma_limit) >=
> > + dma_direct_get_required_mask(dev))
> > + return true;
> > + }
>
>
> Why not do this in dma_map_direct() as well?
Mostly beacuse it is a relatively expensive operation, including a
fls64.
> Or simply have just one dma_map_direct()?
What do you mean with that?
> And one more general question - we need a way to use non-direct IOMMU
> for RAM above certain limit.
>
> Let's say we have a system with:
> 0 .. 0x1.0000.0000
> 0x100.0000.0000 .. 0x101.0000.0000
>
> 2x4G, each is 1TB aligned. And we can map directly only the first 4GB
> (because of the maximum IOMMU table size) but not the other. And 1:1 on
> that "pseries" is done with offset=0x0800.0000.0000.0000.
>
> So we want to check every bus address against dev->bus_dma_limit, not
> dev->coherent_dma_mask. In the example above I'd set bus_dma_limit to
> 0x0800.0001.0000.0000 and 1:1 mapping for the second 4GB would not be
> tried. Does this sound reasonable? Thanks,
bus_dma_limit is just another limiting factor applied on top of
coherent_dma_mask or dma_mask respectively.
