On 5/3/21 9:17 AM, Logan Gunthorpe wrote:

quoted

Returning a "bridge distance" from a "get map type" routine is jarring,
and I think it is because of a pre-existing problem: the above function
is severely misnamed. Let's try renaming it (and the other one) to
approximately:

      upstream_bridge_map_type_warn()
      upstream_bridge_map_type()

...and that should fix that. Well, that, plus tweaking the kernel doc
comments, which are also confused. I think someone started off thinking
about distances through PCIe, but in the end, the routine boils down to
just a few situations that are not distances at all.

Also, the above will read a little better if it is written like this:

	ret = xa_to_value(xa_load(&provider->p2pdma->map_types,
				  map_types_idx(client)));

	if (ret == PCI_P2PDMA_MAP_UNKNOWN)
		ret = upstream_bridge_map_type_warn(provider, client, NULL,
						    GFP_ATOMIC);
	
	return ret;

quoted

   }

I agree that some of this has evolved in a way that some of the names
are a bit odd now. Could definitely use a cleanup, but that's not really
part of this series. When I have some time I can look at doing a cleanup
series to help with some of this.

I'm OK with doing cleanup later. I just tend to call it out when I see it.

quoted

quoted

   static int __pci_p2pdma_map_sg(struct pci_p2pdma_pagemap *p2p_pgmap,

@@ -877,7 +884,6 @@ int pci_p2pdma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
   	case PCI_P2PDMA_MAP_BUS_ADDR:
   		return __pci_p2pdma_map_sg(p2p_pgmap, dev, sg, nents);
   	default:
-		WARN_ON_ONCE(1);

Why? Or at least, why, in this patch? It looks like an accidental
leftover from something, seeing as how it is not directly related to the
patch, and is not mentioned at all.

Before this patch, it was required that users of P2PDMA call
pci_p2pdma_distance_many() in some form before calling
pci_p2pdma_map_sg(). So, by convention, a usable map type had to already
be in the cache. The warning was there to yell at anyone who wrote code
that violated that convention.

This patch removes that convention and allows users to map P2PDMA pages
sight unseen and if the mapping type isn't in the cache, then it will
determine the mapping type at dma mapping time. Thus, the warning can be
removed and the function can fail normally if the mapping is unsupported.

Let's add some of those words to the commit description, perhaps, it's nice
to have. Obviously a minor point though.

thanks,
-- 
John Hubbard
NVIDIA