Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device

From: Christoph Hellwig <[email-masked]>
Date: Mon, 23 Mar 2020 09:37:05 +0100

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.