Re: [PATCH 04/16] PCI/P2PDMA: Refactor pci_p2pdma_map_type() to take pagmap and device

[John Hubbard]

On 5/3/21 9:30 AM, Logan Gunthorpe wrote:
> On 2021-05-02 2:41 p.m., John Hubbard wrote:
> > On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
> > > All callers of pci_p2pdma_map_type() have a struct dev_pgmap and a
> > > struct device (of the client doing the DMA transfer). Thus move the
> > > conversion to struct pci_devs for the provider and client into this
> > > function.
> >
> > Actually, this is the wrong direction to go! All of these pre-existing
> > pci_*() functions have a small problem already: they are dealing with
> > struct device, instead of struct pci_dev. And so refactoring should be
> > pushing the conversion to pci_dev *up* the calling stack, not lower as
> > the patch here proposes.
> >
> > Also, there is no improvement in clarity by passing in (pgmap, dev)
> > instead of the previous (provider, client). Now you have to do more type
> > checking in the leaf function, which is another indication of a problem.
> >
> > Let's go that direction, please? Just convert to pci_dev much higher in
> > the calling stack, and you'll find that everything fits together better.
> > And it's OK to pass in extra params if that turns out to be necessary,
> > after all.
>
> No, I disagree with this and it seems a bit confused. This change is

I am not confused here, no. Other places, yes, but not at this moment. :)

> allowing callers to call the function with what they have and doing more
> checks inside the called function. This allows for *less* checks in the
> leaf function, not more checks. (I mean, look at the patch itself, it
> puts a bunch of checks in both call sites into the callee and makes the
> code a lot cleaner -- it's removing more lines than it adds).
>
> Similar argument can be made with the pci_p2pdma_distance_many() (which
> I assume you are referring to). If the function took struct pci_dev
> instead of struct device, every caller would need to do all checks and
> conversions to struct pci_dev. That is not an improvement.


IMHO, it is better to have all of the pci_*() functions dealing with pci_dev
instead of dev, but it is also true that this is a larger change, so I
won't press the point too hard right now.

The reason I commented was that this refactoring goes in the opposite
direction that I would be going in, if I were to start "improving" this
part of the kernel, via refactoring.

Anyway, I'll leave it alone.

thanks,
--
John Hubbard
NVIDIA