[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