Re: [PATCH 11/14] PCI/P2PDMA: dma_map P2PDMA map requests that traverse the host bridge

[Christoph Hellwig]

On Wed, Jul 24, 2019 at 09:58:59AM -0600, Logan Gunthorpe wrote:
> 
> 
> On 2019-07-24 12:32 a.m., Christoph Hellwig wrote:
> >>  	struct dev_pagemap *pgmap = sg_page(sg)->pgmap;
> >> +	struct pci_dev *client;
> >> +	int dist;
> >> +
> >> +	client = find_parent_pci_dev(dev);
> >> +	if (WARN_ON_ONCE(!client))
> >> +		return 0;
> >>  
> >> +	dist = upstream_bridge_distance(pgmap->pci_p2pdma_provider,
> >> +					client, NULL);
> > 
> > Doing this on every mapping call sounds expensive..
> 
> The result of this function is cached in an xarray (per patch 4) so, on
> the hot path, it should just be a single xa_load() which should be a
> relatively fast lookup which is similarly used for other hot path
> operations.

We don't cache find_parent_pci_dev, though.  So we should probably
export find_parent_pci_dev with a proper namespaces name and cache
that in the caler.

> > 
> >> +	if (WARN_ON_ONCE(dist & P2PDMA_NOT_SUPPORTED))
> >> +		return 0;
> >> +
> >> +	if (dist & P2PDMA_THRU_HOST_BRIDGE)
> >> +		return dma_map_sg_attrs(dev, sg, nents, dir, attrs);
> >> +	else
> >> +		return __pci_p2pdma_map_sg(pgmap, dev, sg, nents);
> > 
> > Can't we organize the values so that we can switch on the return
> > value instead of doing flag checks?
> 
> Sorry, I don't follow what you are saying here. If you mean for
> upstream_bridge_distance() to just return how to map and not the
> distance that would interfere with other uses of that function.

The point is that in the map path we don't even care about the
distance.  I think we should just have a function that returns the
P2PDMA_ values from the xarray (maybe also store it there as two
values, but that isn't quite as important), and get rid of even
the concept of distance in the map path. e.g.:

	switch (pci_p2pdma_supported(pgmap->pci_p2pdma_provider, client))) {
	case P2PDMA_HOST_BRIDGE:
		return dma_map_sg_attrs(dev, sg, nents, dir, attrs);
	case P2PDMA_SWITCH:
		return __pci_p2pdma_map_sg(pgmap, dev, sg, nents);
	default:
		WARN_ON_ONCE(1);
		return 0;
	}