Re: [PATCH 03/16] PCI/P2PDMA: Attempt to set map_type if it has not been set

[Don Dutile]

On 5/2/21 3:58 PM, John Hubbard wrote:
> On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
> > Attempt to find the mapping type for P2PDMA pages on the first
> > DMA map attempt if it has not been done ahead of time.
> >
> > Previously, the mapping type was expected to be calculated ahead of
> > time, but if pages are to come from userspace then there's no
> > way to ensure the path was checked ahead of time.
> >
> > Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> > ---
> >   drivers/pci/p2pdma.c | 12 +++++++++---
> >   1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> > index 473a08940fbc..2574a062a255 100644
> > --- a/drivers/pci/p2pdma.c
> > +++ b/drivers/pci/p2pdma.c
> > @@ -825,11 +825,18 @@ EXPORT_SYMBOL_GPL(pci_p2pmem_publish);
> >   static enum pci_p2pdma_map_type pci_p2pdma_map_type(struct pci_dev *provider,
> >                               struct pci_dev *client)
> >   {
> > +    enum pci_p2pdma_map_type ret;
> > +
> >       if (!provider->p2pdma)
> >           return PCI_P2PDMA_MAP_NOT_SUPPORTED;
> >   -    return xa_to_value(xa_load(&provider->p2pdma->map_types,
> > -                   map_types_idx(client)));
> > +    ret = xa_to_value(xa_load(&provider->p2pdma->map_types,
> > +                  map_types_idx(client)));
> > +    if (ret != PCI_P2PDMA_MAP_UNKNOWN)
> > +        return ret;
> > +
> > +    return upstream_bridge_distance_warn(provider, client, NULL,
> > +                         GFP_ATOMIC);
>
> 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.
+1. didn't like the 'distance' check  for a 'connection check" in the beginning, and looks like this is the time to clean it out.
:)

> 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;
>
>
> >   }
> >     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.
>
>
> thanks,