linux-kernel - Re: [PATCH 04/16] PCI/P2PDMA: Refactor pci_p2pdma_map

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Date:   Mon, 3 May 2021 10:30:21 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     John Hubbard <jhubbard@...dia.com>, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 04/16] PCI/P2PDMA: Refactor pci_p2pdma_map_type() to take
 pagmap and device



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
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.

Logan